I've come from o2 on a Nexus One to Vodafone on the S2 - night and day difference for me in terms of coverage.
Google UK mobile phone coverage - there's a site which brings the coverage maps for all of the major UK mobile providers on there. I've found it's fairly accurate.
In terms of the phone, I miss the trackball from the N1 for editing text, but TouchWiz hasn't really got in the way for me. I only used Kies to check for a software update when I first had the phone, not been near it since.
The screen is a joy to use, and after the N1 issues in daylight, it's so nice to be able to actually read it! The fact the S2 has the noise cancelling as well like my N1 is welcome too.
Another annoying niggle is that maps when the phone's docked in the car charger don't go into "night mode".
My phone's protected under insurance so never had screen protector (remember the sammy has gorilla glass), just got the Samsung leather pouch/case thing - simple but does the job.