Not as perfectly solid as the original
There are six worlds, all with different themes. Some of the stages are easy-peasy (almost too easy) and some are maliciously difficult (if you want a perfect score, that is). If you get a 100 points on all of the stages, something quite good will happen to you! That's the best thing about this game, it doesn't end just because you've saved Luigi. And even if you're done with the game (a 100 points on all of the stages), it's still great fun to play them again.
I haven't beaten this game yet (although I beat the original countless of times), bought it a couple of days ago. I'm already at one of the last stages in the sixth world, so I very much now know how the content differ from the SNES-classic. For example, Yoshi has got a 'voice' in the GBA-version, some of the enemies sound different than what they used to sound like and there are six new stages - one in each of the worlds. If you have played SSBM (Super smash brothers melee), you will recognise this side to Yoshi's voice. I don't have too much trouble with this new voice, but I preferred the old one. The colours of some of the Yoshis are lighter than what they used to be, you especially can tell this by looking at the pink and the red Yoshi. Sometimes the stages lag, when there are too many moving enemies/objects, and after you've touched a 'fuzzy' (you can find those in two of the stages).
All that's mildly annoying, but the most annoying feature to this game are the music and the sounds. I absolutely love the original sounds and the original music, don't get me wrong, but the GBA-sounds are shrill sometimes and it's a shame when you know how lovely it ought to sound. The graphics are nice, although it's a bit pixelish (it doesn't look like smooth lines have created it all, if you understand my point). Maybe it's because I play this game on my DS? Don't know. You can tell this by looking at Naval Piranha and some of the larger enemies/objects. No issue for me though. The former controls have changed, since this is a GBA-game (on the GBs and the GBAs there were only two circular buttons you could perform action with, along with the two L and R-buttons), so it's hard to perform some tricks that were much easier with the SNES controller. That's one thing that I love about the DS, there are more buttons which means more varied combat techniques.
I have so many precious memories associated with this game, I love it (and Yoshi), I wish I could give this a perfect 10... but that'd feel wrong (I gave that 10 to the original instead). Anyway, I'm immensely happy this GBA-game exists which means it didn't become forgotten after the retirement of the SNES. If you haven't played it, please try it! It's the best Super Mario game in my opinion, and you won't be disappointed (if you haven't experienced the bliss of the SNES-version).