I've only had mine a day, but I'm not getting any lag. Someone else said they had some problems with multitasking. I'd like to hear about more tests on that. The only multitasking I tried so far was to play some of my mp3's and surf the internet a little (on wifi), and I had no hiccups on that either. It was very smooth. Another thing, I think some people think there's lag on swiping from screen to screen. I thought this initially, but then I figured out it is just the very precise way the OS handles the touch of the screen.
It's kind of hard to explain, but when going from screen to screen, it doesn't start flowing on it's own until your finger lifts of the screen. So, you could move the screen very fast for like an inch with your finger, then lift up, and then it moves the rest of the way on it's own which is slower. So, that change in speed looks kind of like lag. Once I understood this I really liked it because you can move screens around so fast. I'm not positive, but I think the iphone has a limit on how fast screens move, no matter how fast you swipe. This seems to be only limited to how fast you move your finger.