A Mario RPG on the Gameboy Advance

This game is the spiritual sequel to in the Super Mario RPG / Paper Mario series, again albeit made by yet a new team. However, rather than on the Gamecube, Nintendo have made the decision to have it made for the GBA.

Graphics
Let's deal with that step-down first, because it makes a big impact on the game. Gone are the 3D backgrounds, and in their place is a 2D Zelda-like world. This has the effect that a lot of the game plays a lot like Zelda would. Paper Mario had a lot of Zelda influences anyway, however part of what Paper Mario was has been stripped away.
Seeing as the characters in Paper Mario were 2D anyway, the graphics for the characters are pretty much on par, if more pixely. This is more than made up for by the effort put into animating the characters and battles, which really shows. Sprite fans will probably like, and even I'm impressed.
The backgrounds come off pretty badly though. Rather than the 3D fields from Paper Mario, you have a Zelda style world with objects copied and pasted everywhere. And despite all that it's still a lot smaller than the world in Paper Mario, and a lot less pretty.

Sound
Typical Gameboy fair. Done well, but it couldn't have been that hard to do.

Gameplay
This is where this game really shines. The team have obviously looked hard at the battle system in Paper Mario, and rather than just copy it they thought how they could improve it. And then had a flash of genius. They've added a very clever dodge/counter attack system. Rather than stand stock still and take damage from an attack, you can control your character throughout the attack. You can try to avoid anything heading your way by jumping, then later using a hammer as well. The clever thing is that each different attack pattern usually has a clue somewhere in it, which if figure out, learn the attack pattern and get the timing right, you can dodge the attack and possibly even counterattack.
Another nice thing is that rather than just Mario and a secondary character for special moves, you now control Mario and Luigi simultaneously with the A and B buttons.
The other thing that has changed are the aforementioned Zelda-style worlds and dungeons. You have a lot more abilities at your disposal than in Paper Mario, and there are a lot more puzzles, at the cost that the puzzles are more restricted in 2 dimensions.

This all adds up to a very good game, for at least the first 10 to 15 hours. Unfortunately, it rather lets itself go in the last five. In Paper Mario, as you advanced in the game, a lot of sub quests opened up, and you had the very clever badge system to allow you to customise your character to hell and back, opening up a lot of tactical options.
There is none of that here. Sidequests are minimal. You can equip 3 different types of accessories, however they are usually of the form of progressively more powerful equipment and offer a lot less tactical options.
Even worse, towards the end, the battles are no where near as well balanced as they were in Paper Mario. At the start, it's possible to go through the game, almost without opening your inventory and handing out healing items. However, near the end, enemies have overly high HP and harsh attacks, meaning that the only way to win is to be cheap with items, rather than through pure skill. Much like many other RPGs, however Mario and Luigi nearly managed to get away from that.

Reviewer's Bias
Mildly for

Conclusion
So what do we have here? Well, we have quite a good game, and the GBA is crying out for decent new material in my opinion, so that is all good.
However, as for comparing it to Paper Mario, despite the improvements it makes, ultimately I feel it falls behind. If I were reviewing Paper Mario I'd give it a 9/10. Mario and Luigi probably deserves a:
8/10
If you loved the SNES/Gameboy Zeldas, and you liked Paper Mario, then you'll love this.