I think my signature and avatar says it all. But I digress...
Jet Force Gemini is THE absolute best game Rare has ever made.. hands down. As Dasmos mentioned, it has a fantastic soundtrack which created some very intense moments in the game, some of the best 3rd-person shooting I've ever played (and I LOVE 3rd person shooters -- my fav genre, actually), and characters (including the Tribals) that are easy to fall in love with. I'll even go so far as to admit that I slightly teared up at the end when Floyd bites the dust to save the world. Crazy!
Did I mention the gun variety yet? THE GUNS! Holy smokes... the guns! Tri-rocket launcher is still one of my favorite video game weapons of all time, and the other guns just add to the fun of the game. I love how exaggerated the machine gun gets when shooting... shots fire all over the place. I love it!
And the multiplayer is great, too! It by no means is GoldenEye or Perfect Dark, but it's really fun nonetheless. My brother and I spent countless hours as kids playing the multiplayer. And there's even a little bit of co-op in the single player story, too! There were even unlockable cheats, too! Paintball mode! w00t.
Overall the game has a great atmosphere that I have yet to see replicated to this day. Although, Uncharted 2 came really close. And yes, collecting the Tribals can be a chore, but as long as you get yourself acquainted with the controls, the gun types, and staying defensive... it becomes second nature saving the Tribals (especially the ones that require clutch, last-second saving).
And to end this mini-review on JFG, the game's graphics were astounding for the N64 era. Lots of detail from top to bottom was found no matter what angle you were looking at. The guns all looked incredibly unique, and all had neat animations to go along with them. Graphical textures in the land area were good, too. The game shined with lots of detail during those days.
Besides... how could you not love collecting the head of a fallen bug foe?!
Jet Force Gemini (N64) - 9.1/10