I'm running 5.01 on one machine and 5.5 beta on another. Basically 5.01 is 5.0 with the bugs fixed...and quite a good job too - very stable whereas 5.0 was a joke. I installed 5.01 off a mag cover CD.
IE 5.5 has a print preview feature and is quite stable for a beta...apart from that I can't tell the diff between 5.1 and 5.5.