Doom II returns in it's best console port yet.
Also, forget anything you've read or experienced with Doom 1 on the GBA. Doom 1 and Doom II should be considered distant cousins at best since Doom II easily is the superior game (in every way).
Gameplay wise it play just like Doom II on the PC. Everything is here from the hordes of monsters (minus a boss or two) to the dark moody corridors that we all know and love. The controls are slightly tweaked from Doom 1, allowing for a much more intuitive control overall. The back L+R controlls strafe while the A+B buttons handle weapons and weapon switching. Sufficide to say, with these controls you'll be circle strafing the hordes from hell in no time.
Don't assume though that just because it's a port that it''s gonna be a cakewalk. It's just as difficult as it's PC counterpart (if not more so considering you're using a gamepad opposed to a mouse and keyboard).
Now onto the part am sure most are wondering about...graphics. Yes...they have been shrunk down for the GBA. This is a given and while they don't hold a candle to Doom II on the PC, they hold they're own very well and, believe it or not, are actually pretty nice. They're also a signficant improvement over Doom 1 GBA's graphics (significant is really an understatement, they're a generation ahead of Doom 1 GBA's graphics).
As for the sound...it's fairly authentic to the original PC versions. While I'm sure the dev team had to cut some sounds here and there, almost nobody but the most die hard Doom II fans is going to notice.
Overall Doom II is a great game on the GBA and in this reviewers opinion the best port of Doom II to any console. It plays just like Doom II on the PC...just in the palm of you're hands.