Well, I just got my copy today (christmas, obviously), and I must say, the game's not bad at all!

I've only played the first few levels, but I think I can give you a general idea of how the game plays:

I was reading previous posts about control issues, ie: jumping and shooting, and it's true, you can't jump while you're in the middle of shooting, but you CAN shoot in the middle of jumping.

The jump/shoot limitation is due to Jazz's 7 way aiming system, as opposed to JJ1 and JJ2's system that only allowed up, left, right, and crouch shot.

You use B (or A, as default), to fire, and hold up, left, or right, to shoot up, left, or right, and you can shoot diagonally by holding up + left or up + right, down + left, or down+ right. to crouch and shoot, you must hold down, and THEN the fire button. On the same note, if you try jumping while the fire button is held, it doesn't allow it.

As stated in previous posts, the uppercut and buttstomp are missing, but it plays more like JJ1 anyway. The Helicopter ears are achieved by simply hitting the jump button twice, and holding it. no additional pressing necessary.

The AI in the enemies is improved over the PC counterparts, they tend to know how to attack, and take multiple hits (even on normal) to beat. Most enemies are equipped with guns, clubs, etc, and they can shoot in all directions al well. If you're crouching and firing, some of them will do the same!

Although they are smarter, it does take away a little bit of that Jazz "shoot everything that moves, and if you can't shoot it, take it" feel, as you can't plow through them like I so often enjoyed in JJ1 and 2.

The graphics remind me of JJ1, due to the small resolution, and the fact that the GBA's screen is wider than it is tall, so sometimes it's a leap of faith. The animations are well done, if you kill an enemy, he keels over and dies, instead of little bits of him flying all over the place (which is one of the cool things about JJ2.) If Jazz dies, he falls over, gasps for air, and collapses, and if you have more lives, gets up and keeps going. You eventually get used to Jazz's red shirt and jeans, but I still personally prefer the red headband and blue backpack.

The level structure is interesting to say the least, having to shoot targets or kill enemies for keys to open some doors. The tilesets match the level, and the sewer level in NOT an exact copy of the colon tileset, but some tiles from it were reused.

The backgrounds, are quite nice for a gameboy (they're layered like JJ2's).

The items in the game are also different. you collect different guns and ammo, much like in the previous versions, but there's no coins, gems, or other goodies. Your usual 'goodies' are replaced with cold hard cash. Cash by the wad or bucketfull.
Medic packs refill your health, which no longer works on the heart system, but instead a meter, much like in JJ1, but bigger. Carrots, appearantly, are used as grenades, by pressing the L button.

The guns are quite different, and yet, the same:

You have your standard Blaster,

the riochet gun which is kinda like the BB bouncers but you can get hurt if a shot bounces back and hits you,

the machine gun, (which speaks for itself)

the flamethrower, (I'm guessing it's like the JJ2 toaster)

the rocket launcher, (RF missles, anyone?)

and "the big gun", which, from its description, sounds kinda like the UT redeemer. Massively powerful, but also dangerous to the person who fired it.

Well, that's my first impression of JJGBA. Hope it's helped you to form your own opinion of the game.