Sonic's swan song on the GBA is satisfying indeed.
Sa3 takes place sometime after the events of Sonic Battle. Eggman has fragmented the world via Chaos Control. The only kink in his armor? The unbeatable duo of Sonic and Tails. There's a small twist ending pertaining to G-mel, Eggman's newest creation built from the scraps of Sonic Battle's Emerl, but the game ultimately doesn't aim for anything phenomenal.
The game finely balances the speed of Advance 2 with the exploration, branching paths, and fun that is associated with its Genesis ancestors. Surprisingly enough, Sa3 decided to bring back a feature from Knuckles Chaotix that actually pays off; the tag-team. Instead of entering the stage with just one character, you get to pick two. The various character combinations affect your team's abilities which determine how easily you traverse through Acts or trump a boss. Each individual character has their own "Tag Action". By holding the R button and then releasing it after a short charge, you can access your partner's own abilities, which usually differs on the land and in the air (For example, Knuckles' air action allows you to ride on his back while gliding, while Cream's allows you to double jump while gaining control over Cheese). While at first you only can play with Sonic and Tails (nostalgic, right?), the ranks of playable characters grows to include Knuckles, Amy, and Cream. With that many different character combinations and tag actions, Sa3 has a level of depth rarely seen in other games in the series.
The crisp 2D graphics are, like in the other games, pleasing. They're nothing special, but are (in a way) vaguely similar to that of the Genesis titles.
The music is pretty good, but nothing spectacular. There are some nice remixes, including one of Sonic 1's Green Hill Zone. As usual, it's nice to listen to.
Unfortunately, this is the last game where the original VAs voice their respective characters. While there are only a few grunts and "Yeah!"s, I still miss these guys. R.I.P. guys...
It's a satisfying end of a trilogy of exceptionally good 2D Sonics. It's no Sonic 3 & Knuckles, but very few 3D Sonics nowadays can even touch this one.