WARNING - Several problems after update - WARNING

Short battery life right after the update is normal.  it'll improve in a few days.
Android is an intelligent OS, which constantly adjusts itself to how the phone is being used.  Since the phone now has been flashed to install the updated OS, it needs to figure everything out all over again.  I had the same problem after updating, but 3 days later it was already better than with 2.1

As for the other problems, several people had various issues, which I suspect are caused by settings for 2.1 which don't quite work with 2.2 . 
I make the same suggestion to all of them : Make a backup of your contacts and all personal data, and do a factory reset.  So far only one person hadn't been able to sort the problems that way, and I'm suspecting that that person simply loaded the backup of his old settings, which ruined it again.  So don't do that, just set everything up from scratch.

Yes, you'll need to re-enter all contacts and passwords, and you'll need to download all your apps all over again.  It's a major inconvenience, but it's not the end of the world. 

This isn't HTC's fault really.  I'm not sure if it's Google's fault either.  Many people don't have a problem at all.  In my case it was only the camera which simply wouldn't work anymore.  Everything else worked perfectly fine.