One of the best handheld games money can buy.
Graphics: Simply lovely. The cartoonish style suits Yoshi perfectly, and the crayon-esque backgrounds make the island look like something that sprung out of our imaginations as kids. This game looked superb 15 years ago on the snes, and looks superb now on the gba. The animations are all well-done, and the little things, like the running and flutter jumping sprites, make this only make it all better.
Music: Do I need to say anything? This is Nintendo music at its finest, from the laid-back melody of the first level to the energetic athletic theme that plays during my favorite level (the skiing one!!!). It has been improved from its predecessor to include better-sounding instruments and more of them, making a game with already great music sound even better.
Sound design: Great. The fact that they added in voice acting for Yoshi is wonderful, as now we can hear the trademark "maaaouugh" whenever we flutter-jump or the "hup!" whenever we jump. However, they also added voice acting for baby mario. They made it so players would try their hardest to get back baby mario whenever he is lost, by making him cry whenever he falls off yoshi. It's annoying, but it serves its purpose.
Gameplay: Awesome! Unlike traditional run & jump mario games, you can actually turn enemies into weapons (note that I don't count super mario bros 2/doki doki panic as a traditional mario game) by swallowing them and turning them into eggs, aka high-speed ricocheting projectiles. This adds a whole new level of depth to puzzles and battles never before seen in a mario game, and is very well-implemented. The platforming itself is some of the best in the mario series, and is widely varied. There can be levels filled with platforms that begin falling as soon as you step on them, the platforms that immediately start moving once you step on them, and the traditional scrolling level for the old style. However, new styles are added, such as using ghosts with platforms as heads to traverse tricky courses, using eggs to create paths, and my favorite: skiing. That's right, there is a skiing level, and it is awesome!! In total, the gameplay rocks. In addition, the replay value for this game is great, as each level has a number of flowers, red coins, and stars to collect, which add up to a total score that can go up to 100, the maximum possible. Trying to get 100 on every level is a very difficult venture, and may have some playing for weeks.
Overall: if you want to experience the quality that made Nintendo such a great and well-respected company in the late 80's/early 90's, then this is the game for you. Buy it.