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No problems for me on my iPhone 4 and iPad 2 and I think it's been the best and most stable release of iOS so far.

I'm sticking with MobileMe for as long as I can so I'm not using iCloud or iTunes Match, which might be one of the reasons I've had no problems. :)
 
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iOS 2 was really bad!

Ios5 is fine for me except the call history bug!

iPhone OS 2 (as it was called then) was fine for me. iOS 4.0 on the other hand was the buggiest piece of crap I have encountered.
 
Upgraded my private iPad2 and iPod touch without any issue; works without the mentioned issues. Mail against Google,MS, ICloud and own IMAP server. No real issues here besidethat my own IMAP sometime delay delivery (shared hosting ...)

My company iPad 1 is also up to 5 and connected to exchange server; no issues here too.

My iP4 still on old iOS 4.
 
here are some of the problems i've experienced so far:

1) battery-drain
2) emails not coming into the Mail app sometimes. Won't be notified until I randomly go into the app and realize that the Mail app is frozen and have to restart to get it to connect
3) 3G connectivity issues - show full bars on 3G but constantly times out. all i see it that damn wheel spinning and nothing happening.

1, can be caused by the software/hardware doing more things.
2 & 3 have been around since the beginning of the iPhone. Trust me, I know.. I've had every one of them. Gmail, Yahoo, iCloud... randomly doesn't push emails, although most of the time my iPhone gets emails before any of my other email clients currently open do. 3g connectivity...um... ATT????
 
It works pretty well. With the massive push by Apple to build millions & millions, it's only normal for problems to occur. As long as it is accepted by the masses, that's good enough.
 
Since restoring from the iCloud on New Years Eve it is more stable. Sometime Safari will crash but usually on non mobile sites especially ones loaded with ads and social media buttons. The App Store will sometimes crash too but that has been unstable even in iOS 4.

As for apps any that keep crashing I delete.
 
My iPhone 4 is still on 4.3.5. I won't download it because of all the problems I've read about it. The battery drain already increased noticeably with the 4.x.x releases. Don't remember which one really triggered it. But I can't afford for my battery to drain any faster than it already is.

I downloaded ios5 to my iPad and went from a battery that seemed like it lasted forever to a battery that I'm having to charge every couple of days. Really, really sharp decline in performance.

Very disappointed that this hasn't been resolved and, once again, still seems to be a case of Apple saying, "well not EVERYBODY has the problem, so there's not really any problem to address." The ios version of "you're holding it wrong."

Frustrating.
 
The problem is that Apple makes it nearly impossible to downgrade to a older OS. At least on the PC don't know about Mac's OS you can always downgrade a driver or software if it causes problems.
 
The battery drain seemed to be the biggest thing for me. But I also have chargers wherever I'm at (home, car, work PC).
 
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