So far, with the exception of a couple of plugins I use needing to be updated, I'm finding 4.5 to be fairly issue free.  There's a tad bit of lagging when viewing content; but we're talking a couple of seconds.

I'm loving how autofit now works with gen 4 content w/o nuking all the morphs.  No more going through the nightmare of converting to triax and then going through the transfer utility.  The smoothing modifier works a lot better now.

I don't keep my scenes for much longer than it takes to render them; so, loading old scenes is a non-issue for me.  TBH, I haven't dug as deeply into the new release to see if I could find anything that's broken.  For what I want it to do, it works like a charm and much smoother than 4.0 did, so I haven't had the opportunity to attempt to push it to breaking point.