GigaOM has performed a benchmark analysis of iOS 5 on each device using GeekBench 2, Gauge Mathematical Tool, BenchTest, SunSpider, and V8. They found that except for the iPad 1 iOS 5 brings significant improvements to the performance of your device.
In fact, with the exception of the original iPad (the older device that was tested), all devices actually gained in performance in most areas. That is simply amazing for the first release of a major OS update. This is nothing like the 4.0 release, where we saw incremental performance gains over the lifetime of 4.0 to restore what was lost to older device; iOS 5.0 is a solid release for all where performance is concerned.
I love Apple but! I updated my iPhone 4. Everything was ok but I found one bug I think. Music app... When I'm going through covers it gets stuck, like slowing down and then force quit. It shuts down by it self... Anybody has something like this? Oh, and I lost few covers from my albums...