iPhone 3GS performance impressions:
As someone who consider IB1 to be only barely playable on the 3GS, IB2 is definitely playable. The framerate is way more stable and has alot less stuttering. That said the graphics are scaled down massively compared to what the game looks like on the iPad 2/iphone 4S. All the cool effects are not present. It looks like IB1 with maybe slightly worse texture quality.
I am not complaining tho, it is still a technical marvel to see a game like this running well on a 2 years old+ phone. I also dont play that many games on my phone but i found IB1 and Battleheart to be awesome, so i caved for IB2.