The following review is based on the first beta of iOS 5, so the negative parts of it apply only to this build and not the final product.
I am using it for 24 Hours so here is what i found:
1. Battery drain because of iCloud sync (battery drops twice faster than normal)
2. Memory leaks
a)iMessage when combined with notification center
b)Phone app when you use custom vibration
c)System wide leaking which causes all kinds of applications crashes for no reason (or for low memory reason)
3. Bad memory management - closed apps do not free all the memory they used while they run (which is one of the reasons that we have low memory crashes all the time)
4. iBooks crashes (probably there is some incompatiblity with Newsstand because they share similar technology)
5. Newsstand does not work (there is no beta market for it yet)
6. Wallpaper does not work (either wallpapers are missing or it does not work)
1. Sounds - you can manually setup almost all the system sounds
2. Custom vibration - one of the best features which no other phone have
3. Notifications are awesome - notifications in iOS5 is very clever implented and are actually useful
4. Camera in lockscreen - this is another amazing feature. I wanted to photo something but i lost time to unlock the phone and open camera app and in most ways i missed the moment for picture, now i can do it far faster with this new addition.
5. iMessage - i feel it is kind of ripoff WhatsApp but it works really fast
6. Reminders are not so much of a feature because you can do similar things with calendar but it is good implentation with very simple interface.
7. Long awaited edit option in pictures app is awesome
8. Camera shutter with volume up button is too a great feature
9. Notifications on lockscreen are great as well
So far this is what i had in mind, probably soon i will update this list.
iOS 5 is a must have upgrade because it saves me time to install many apps in order to do some things - now everything is included. I do not advice regular users to go for the beta because like i said it is unstable (stable for a beta but still unstable for daily use)