PSA: This is a prequel; please watch AFTER you watch the original series
As far as how it adapts the story, it does a very good job adapting the prequel story-pieces of the manga adaptation, "The Origin," and I definitely recommend it to anyone who has seen the original series, especially if they're about to re-watch it.
If you consider the early-UC timeline to be "The Story of Amuro Ray and Char Aznable," then this sets the stage perfectly: It takes you from "before their time," through their childhood and adolescence, and finishes you off perfectly with all of the main players taking their places for the beginning of the first episode of the original series. It definitely focuses more on Char than it does on Amuro, but, looking at the early-UC timeline, Char IS the character whose story affects the world the most while his past is the one least explained.
I'll put it bluntly: the animation flips back and forth between "cheap horseshit" and "a little bit more than above average," and that definitely detracted from it. If none of the CGI had been used (or if any actual effort had been put into the CGI), then this would not have been anywhere near as big of an issue. It's embarrassing that something that was supposed to be this huge production ended up with scenes that looked as bad as this did.
The sound design, on the other hand, is absolute perfection -- It's everything I could've wanted. The Japanese voice acting was what it was, to be expected, and the English dub was pretty good, though I would've liked it more if more of the original series cast had been brought back (especially for Amuro, Char, and Bright).
Like I said: If you've seen the original series, watch this, especially if you're about to re-watch it. But do NOT watch this before watching the original series.