Whenever I finished FMA Brotherhood, I've wanted to watch this version as well, but I always thought it was too soon to watch essentially the same thing again, so I held off and forgot about it again until the itch for FMA came back and I watched Brotherhood yet again, repeating the same loop.
This time I finally went ahead and gave it a chance, and I'm sure glad I did. Even though the 2003 version has a somewhat bad rep, I can say I enjoyed my time watching this version, and it wasn't boring at all to watch, because it strongly differed from Brotherhood essentially from the start - there wasn't a single episode I could've skipped without missing something.
Let's start with the art: This version is dated, and that's obvious. No matter where I looked it up, the resultion was as expected from a 2003 anime everywhere, and the frame ratio is unfamiliar and somewhat offputting nowadays (unless you recently rewatched ATLA or another medium in that aspect ratio like I did and were still used to it :P)
I've heard many times that Brotherhood essentially rushes through the first few episodes, expecting you to already know what happened in the 2003 version - however, I actually had the opposite experience: I never had any trouble following the storyline in Brotherhood, not even on my first viewing, yet I was VERY confused with the first 10-ish episodes of the 2003 version and would've probably given up if it wasn't for my Brotherhood knowledge filling the cracks and enabling me to put together the puzzle somewhat coherently.
The 2003 version offers a lot of new details and insight and indeed has a somewhat longer path to follow along the key events that both animes share, however I found it very hard to follow what was going on, even with the previous knowledge of Brotherhood. The first few episodes in particular introduce a bunch of plotpoints all at once that are stretched out further in brotherhood, however it omits key poiunts to the worldbuilding and how the universe works that would've been essential for me to follow the story, had I not known them from Brotherhood.
Additionally, the first episodes include timeskips that the audience is barely made aware of and last over several episodes, making me get completely lost on where exactly in the story I even was at some points - in the beginning I was unaware if things I was seeing at the moment happened before or after what I'd seen in the prior episodes. But sticking with the anime through that pays off!
While the timeline of when what happens is pretty different from Bortherhood and the beginning of the 2003 version felt rather rushed and confusing to me, after the frist few episodes it starts slowing down considerably compared to Brotherhood, leaving more time for certain developments.
I've heard numerous times how people say you can combine watching the two animes to get the best of both worlds, but I'd definitely disagree on that! Both versions differ strongly right from the start and key plotpoints are laid out completely differently, which leads to very different outcomes.
Pretty early into the anime, I've essentially started viewing it as a parallel universe or cinematically realized fanfiction, rather than a different version of the same story, because the laws of the worlds, the characters and their relationships are far from interchangable between the versions - it really is essentially taking the rough concept and characters as a base to tell a new story of its own.
As for the characters: In this version I really got the impression that the brothers are truly kids, while in Brotherhood I often felt like they were way older than they started out in canon. The 2003 version feels more lighthearted and over the top at times (especially in the beginning where the comedic faces fmab was often critizized for stuck out to me heavily).
Several characters get way more screentime than in Brotherhood, which was a pleasure to see, however unfortunately I have to confess that I wasn't able to build a connection to any of the characters in this version and didn't cry once, even though I felt as if this version gave me more time to get attached to certain characters - yet their fates didn't hit me, even though they still do when I rewatch Brotherhood for the xth time. Might be that the vibe of this version isn't suited to my emotional level.
I realized at many points in the story that this version felt less immersive to me, so while Brotherhood kept me on the edge of my seat and throughoutly invested, the 2003 version often felt like it was just going through it's motions as an entertainment medium should. Somehow the reactions characters had to what was happening around them didn't feel as "real" and relatable to me, which often reminded me that they were just chracters and made me myself feel a lot more grounded in my experience, while in Brotherhood I often lost myself in the story.
Apart from the characters feeling different, there was also a more fantastical element to this version, while Brotherhood felt more realistic. Several scenes early in the anime in particular (The priest fight, Scars interaction with the research, Lyras introduction) made alchemy seem more magic-based than science based in this version as compared to Brotherhood. This isn't necessary a bad thing though.
Initially starting into this show I was worried it could dampen my mood after finishing Brotherhood a 5th time, as this version has a reputation of having a worse plot, due to running out of source material. However, I still enjoyed this version plenty - it added a lot of fun new details and "what ifs" that I had a blast discovering.
It also added further background info that I didn't know I was misisng within Brotherhood, but that further enhanced certain plotpoints for me (even though I'm not sure just HOW canon certain parts are, since as said: many characters are essentially someone completely else with the same name and look).
Many of these new outlooks also changed the meaning of certain plotpoints significantly, like protagonists actively causing certain tragedies rather than helplessly experiencing them unfold while not being the trigger themselves.
All in all: This was a drastically different show and story from Brotherhood and that from early on - around 20 episodes in you could barely still feel a connection between the two, but as treating it as a a form of "what if", it was still a very entertaining show. I'd recommend viewing it as a completely disconnected medium from Brotherhood altogether.
Certain plotpoints in this version were interesting ideas that I loved figuring out, but there are still a lot of questions left unanswered for me after having finished the anime itself (have yet to watch the movie), and some things just don't seem to add up no matter how I turn them. Also while certain ideas introduced in this version seemed very intriguing, I didn't feel satisfied with the way they were handled in most cases (minor spoiler: apart from the case of Lust in particular).
I feel as if already having seen Brotherhood greatly improved my enjoyment of this version, as it motivated me to stick with it through more confusing patches and see it through to the end. As a standalone, I find it to be a fun time, but I don't think it would've left much of an impression on me by itself.
One thing that I do wanna point out in comparison though, is that I enjoyed how this version forced them to confornt their ideals. I've heard a lot about the 2003 version being the "darker" version of the two, and I didn't necessarily feel that way, but I felt that this version had a slight leg up in that (small spoiler) they had no choice but to take or use lives of others.
While I found myself enjoying the way they handled certain plotlines more in this version than in Brotherhood, overall I felt as if the 2003 story created more questions than it answered. Unfortunately I also couldn't find myself to care much for the characters, even if my favourites were harmed - I felt a certain disconnect from the show throughout the entire runtime and never managed to immerse myself to the point of treating this as more than a potential parallel universe to what "actually" happened.
Brotherhood might spark similar feelings for people who watched the 2003 version first, but I unfortunately wasn't able to view this as anything but a fleshed out "what if".
I felt compelled to give both story and art and therefore also the overall rating a mark lower than I did, but I'm giving out bonus points in consideration of the anime being almost two decades old at this point and having been in the shoes of being the first and the one that had to figure out where to go from the point where the source material was depleted.
