Great game
I love the game, but I do have one large problem with it. I beleive that a "speed up" option should be available, so I don't have to wait for a motionless, still grenade to explode. The "stop grenade" feature helps, but sometimes I'm sitting still waiting for it explode next to an enemy, and in my boredom I hit the space button, trying to speeed the process up, which results in a lost grenade.
I also found that the pipes are kind of gimmicky as well. You have to hit them dead center with the grenade to get the grenade through, whereas it should count if I get anywhere near the pipe. For future entries in the series, you may want the pipes to suck the grenades into them.