This game kills my battery. Even with the C-gear stuff turned off my Dsi is dead after like, 2 and a half hours of play. Has anyone else had this problem?

I've had no problem with it.  Try turning down your DSi's brightness.  If it's at the lowest setting, then your battery must be dieing.

No problems for me.

" No problems for me. "

Only time it went down faster was when the C-Gear was enabled. 

I keep my brightness maxed and am amazed with the long battery life. Than again, I've played so little of my ds in the past 3.5 years that I forgot how awesome the battery life is. I stayed up till 5 last night playing pokemon because I couldn't sleep. 

Yeah, I haven't noticed that problem myself, but I can understand the constant wifi being an increased strain on the battery. I would try playing a different game for an extended period of time and seeing if you get the same results, to see if maybe your battery has started wimping out.

Once I turn off C-Gear it's fine. 
 
Look on the bright side, this is preparing you for the 3DS's battery life! :D

I've noticed it's been going down faster then normal but I'm getting five or six hours out of it.

I kept thinking mine was dying extra fast, then I just realized how long I was playing the game for.

I run my DSi with the brightness on 2 and C-Gear turned off unless I need to use it for something. Never had an issue with battery life. I've had some pretty long stretches of play, too.  
 
I'd check what your brightness is set to. You could also turn wireless communications off at the system level as well (meaning the yellow light on the system is off), that might help a bit more. Not sure if that'll actually increase battery life, though, so don't bite me if I'm wrong. Haha.

If you do all of that and still only get 2 to 2.5 hours of life out of a full charge then there's something definitely wrong, sadly.