I have an iPad3 16gb. My update to ios7 (& all its subsequent points) was without incident. The irritant of the parallax view was soon overcome following the "find" of turning it off in settings. The wallpaper size issue was a pain, but overcome by resizing to the correct size for the iPad.
ios8 has been even smoother - yes the download crawled in (took 7 hours even on a 37mbs fibre connection) - but it was on the actual release day so to be expected. (Although my iPhone 5c got it through in under an hour).
All in all I think 8 is an improvement over 7. I don't really notice any performance difference to be honest. But I have done several resets (power & home) first after the update & also after each "crop" of app updates.
I have noticed some "glitches" which I'll post about in a new thread to see if anyone else has come across them.
Cheers all