"What a great manga! I wonder why it's not popular like One piece?"
I wanted to write those words but I've never fully checked out One Piece. The main character just didn't interest me.
Instead this was taken from a Manga Foxer who made that comment.
So from my perspective, it was more because it was a fresh pirate world. If you like Shonen animes, this manga doesn't have much build up to the fights but it's chock full of elements that makes the world unique enough to be on par with some of the classics.
The characters are a bit hit and miss: being too hot blooded
talented at times and at other times being too reckless and focused on certain characters.
This is made up for the vast personality given to the rest of the cast when it doesn't revolve around fight scenes. The females too are uniquely designed compared to your average shonen. Some designs even able to be foxy without really being your traditional anime/manga beaut.
This is still more of a kiddie light hearted manga though (at least at the point that I've read) but I mentioned the females above because they have the strongest art presence as far as highlighting the unique designs of the world. If the "strong" female lead designs don't pull you in, chances are the rest of the world won't.
As far as premise, I wouldn't suggest this to Trigun fans but let's just say despite being a pirate themed world the crucial plot is similar to that. That is to say, if you're a fan of magical weapons, mystical creatures and wimps that have the capability to go boom then this manga is worth a check as long as you don't coming in expecting a Western themed manga.
The difference between this and Trigun is that you have a pletora of young annoying characters which makes the powers and the character development more...shall we say...annoying. It's like being part of a unique manga world but bringing your annoying nephew or friend (if you are a kid) to the party. Truthfully this should be what makes the manga fresh but it just stumbles because the brats while having a good premise initially never lives up to their development as characters pulled into a bad situation and so you have a ton of predictable volumes where the kids are seriously like normal kids being bailed out by a Superman character.
Other than that, there's not much to be said about this manga except that it would make for one of the best JRPG plots because of the premise that the main characters start at the middle of their journey and many of the interesting characters having grown older which again follows the Trigun plot of rediscovering your past. This is really where the manga starts to shine, if you love these kinds of lighthearted growing old to be a badass plots then even if you hate everything else about this manga (or the youth going on an adventure theme), it's worth at least a 9 just for those scenes.