Here’s a short feedback of Firmware 2.1 after a week upgraded to it on my ‘going to 1 year old’ iPhone:

• Power-on/Start-up seems to take longer than before.
• Backup with iTunes 8 are now significantly faster (less than 2min, I have 2 pages of apps).
• I have zero force restart for this week.
• Battery life ’seems’ to be better. I can’t tell by how much, since I did not perform a proper test. But before this I need charging every 1 1/2 day. Now it can survive till the night of 2nd day.
• Apps are now much more stable. I have zero ‘quit to homescreen’ incident during this week.
• No more lag when you lock the phone.
• Pressing home button are almost bring you to homescreen instantly on most app I tried. No delay.
• Typing long reply on MobileSafari no longer see dramatic slow down.
• SMS app are much responsive than 2.0.x.

That’s all I can remember at the moment. What’s your take on firmware 2.1?