I upgraded today, from 4.1 to 5.1. Big step. The question ist: A big step ahead or...?
What I can say:
- Much better than iOS 5.0.1 (battery issues got a bit better; Coverflow's now 'sort of' smooth.)
- Some problems stayed (laggyness when unlocking with the slider first time you boot; very long shut-down times, even though I thing they got better as well.)
- Booting times are surpisingly low!
4.1 is much more stable, just doesn't has so many bells and whistles. If you have a jb-iPod touch on iOS 5.0.1 with SmoothCoverFlow an some other tricks installed, 5.0.1 ist a bit better I'd say. If your Ipod is not a jb-one iOS 5.1 ist better. I hope jb will come soon for 5.1.
One last thing: I often close my apps in the double-home-button-click-view. This didn't help me at all.
They are programmed to link to your runnig apps but should not use much of you ram when they are not open. They were introduced with 4.0 - not bringing any kinds of problems. I don't think that they cause issues in iOS5. It's something else.
Info: iTunes didn't give me the option to set my iPod up as a new one (without restore). I saw the screen but it was unclickable - in the front there already was the restoring-window. When I canceled it the first time trying, it threw me out of the upgrading-process. But I'm happy with my restore - otherwise I would have lost my gaming scores.