So its my first try at any 4.5 and I got the latest. I'll list my the things I like about it over 4.2:
1. New format of messages is much easier to view/read. even includes the little contact photo I have for many.
2. New alpha fonts are much clearer and better than any prior
3. Some decent browser improvements but can still make leaps and bounds there
4. "Remotely" searching your mailbox... great feature.
5. Setting security color for enterprise messages no longer forces barber shop diagonal stripes but a nice background outside the border. Been wondering why this wasn't like this day 1.
6. video recorder works better than showtime on my jailbroken iphone.
7. "Dimension L" theme now my favorite. combines best of both zens.
8. activity log option on address book contact is cool way to see all activity from that user. very helpful.
9. Docs to go not bad, will be limited but useful at times.
10. All of our (3) in house made apps still work.
11. New divider for address list, either line or color stripes is great addition.

Anyway good stuff, nothing breakthrough but all needed features when devices like the iphone exist now. They should make the 9000 with the intel atom then maybe they'd solve their batt issues.