I use a 3Gs, and I just upgraded to ios4.
I have absolutely no issues, I was apprehensive but everything is working out, no glitches or setbacks of any kind.
I think if you're not using a 3Gs or newer then ios4 may be a bit of a resource hog.
I've played around with a co-workers Android phone, but I wasn't too impressed. On paper it had the iPhone beat, but the OS felt clunky and jittery. Another setback I didn't like was all of the beta applications, or add on features that just made the phone a mess. The phone itself was very cool though, I don't recall what it was called but the display was great, and looked like a quality product.
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XBL Gamertag: Kerpal_CZ