The over all user interface design is very smooth and distinctively Apple, like it or lump it.
I see this OS as more of an evolution of what iOS is going to be. There are some very nice refinements over version 4, such as the Notification Center is a nice addition. I still prefer the Android OS approach to notification of emails/texts/appointments, but iOS 5 has went a long way in the right direction.
Apple has made no secret of hiring some from the “jailbreaking community” that have made “un-official improvements” to iOS. I have seen evidence of this in two places. First is the Notification system. Second is a neat little camera trick. If you double click the home button from a lock screen, you have a nice shortcut to the camera (in addition to getting the top volume button as a shutter button).
Other improvements that rise to the top is much improved twitter integration, small but needed tweaks to the mail and messaging apps, and improvements in the settings app (more ability to change more things!).
I have not noticed any significant drop in speed from iOS 4.3.3 on my iPhone 4.
I have noticed that with heavy use, you can experience a drop in battery life on iOS 5. For most users, this will be almost a non-issue. It is similar to the battery life that you experience when you jailbreak iOS 4.3.3.
I am very pleased in the steps Apple has taken to improve iOS. Is it the “nail in the coffin” for any of its competitors? Not in my opinion. But it is a lot of small improvements to a already very good mobile OS/Hardware combination.