FYI, there's a significant issue when upgrading from LR4 to 5.2, with any images where the spot removal tool was applied: http://feedback.photoshop.com/photoshop_family/topics/lr_5_2_is_not_rendering_correctly_photos_with_spot_removal_applied_in_lr4
It's better in 5.3RC, but the behavior still isn't identical to LR3/4 - so depending on the particular image/edits applied, you might need to replace the previous spot edits.
I do really like the new Spot Removal and Radial Filter tools, though. For me, those alone are reason enough to upgrade, and aside from the hassles mentioned above, I haven't had any issues with 5.2/5.3RC.
I had issues with spot removal being treated differently between version 5 (my first installation) and 5.2. I think it had to do with introducing feathering for the spot removal tool in 5.2. All the spot removal I had done suddenly had feathered edges, requiring much of it to be redone. Thankfully, I had only been using Lightroom for a couple of weeks.
As far as performance goes, I'm on an ancient (six years old!) laptop and manage. I have to be careful not to have too many photos open at once, and the more brushes I use, the more it lags, but I'm getting by. Despite my computer's inadequate hardware, I'm quite happy with Lightroom and can't imagine what I would do without it.
By the way, when I jumped aboard the Lightroom bandwagon, it already had the radial filters and paintable spot removal tool. However, I use those two tools so often that if I had a version without them, I'd upgrade just for those two features.
I hope this was helpful...
PS - Here are my sucky hardware specs :
HP Compaq nc8430 (circa 2007)
Windows 7 Ultimate x64 (Upgraded from Windows XP Pro)
Intel Core 2 Duo T7200 2.0Ghz
4GB RAM (Upgraded from 1GB)
7200RPM HDD (Upgraded from 5400RPM)
It may be old and battle-worn, but with some hardware and OS updates, it has stayed relevant enough for me to use it as my primary work computer nearly every day for six years (including some graphic design, multimedia, et cetera). It even got me through the laptop display resolution drought where everything was offered with a miserable 1366x768 resolution (I love me some wide spreadsheet action ). Obviously, when I eventually replace this workhorse, I'll appreciate the huge jump in performance for things like Lightroom, but I'm doing okay as-is.