I installed 4.5.018 on mine. Works great, love the video recording feature. I also like the 'media' improvements that RIM made as I use my phone as a MP3/video player a lot.
One big question for me was, will the theme I created for OS 4.2 work on 4.5? And it actually works perfectly.
I had it on my 8310. I downgraded because I could not get AT&T Navigator (Telenav) to work. Other than that, I was totally satisfied with the OS (until 4.6 and the 9000.).
If you don't use Navigator, I suggest the upgrade.
Another downside though is the memory hog. Witht the same apps installed, I loss 10mb. I assume its the video/voice recorder. This is without Docs to Go installed.
Posted from my CrackBerry at wapforums.crackberry.com