6.0 was pretty painful
the late 6.3's were getting there
6.6 was a step backward
The current 6.6 release is one I might switch from 6.3 to, if there are no howls of trouble in the next few days.
Having got a qualifying 5.5 Retail and a free 6.x upgrade, I could choose to go back to 5.5 - but I won't. A very good move, actually, as I hate the "Express" interface, and would have been lumbered with it on my new drive - other than that, I was quite happy with the 5.5 (not express) bundled on my old drive.
It's the usual story, even the 5.5.x steps were usually backwards, with the last 5.5.x.x being better than the next digit step (new features, new bugs).
I'd say that I haven't really noticed the difference from 5.5 to 6.3 though, at least, not in the bits that I've used so far - other than for drive support, there isn't much reason to upgrade from 5.5 to 6.