The increase in image quality of LR5 over LR2 is amazing. If you shoot raw, it's like upgrading your cameras to a much better model for a bargain price - no exaggeration. Not to mention the many, many other improvements.
Here's a screenshot of a flawed image [underexposed cat] I use to demo some of the big improvements in process versions, with from left to right the untouched raw file, process version 2003 [LR1 + 2], Process version 2010 [LR3] and Process Version 2012 [LR4+5]. Only the last one looks like the scene as I saw it, when Idiot Cat no. 3 wandered into shot as I was shooting some cloudscapes for my image bank.
This particular shot does not demonstrate how well PV2012 handles highlights compared to previous versions, but it's also very impressive.