FWIW the first time I installed OS5 yesterday I did Restore From Backup. Everything went fine but phone was definitely sluggish esp when typing texts etc, when the text tone and reaction from my touch was quite clearly out of sync with the onscreen execution. I also did notice pretty quick battery depletion tho' not like some people are reporting.
Today, just to celebrate my BB coming back to life after 3 days of NO SERVICE (= fail), I decided to bite the bullet and try a Restore As New Phone. It worked fine but required that I reset all my app icons, folders, settings etc. Basically a major pain in the butt. BUT, the phone is now WAY snappier without question and since I now have a backup based on this new, clean setup, I only have to do it once so it's worth it. Too early to tell if it makes any diff on battery life, can't see why it would.