The latest version is typically the most feature-full one, but what about stability? Working with KDE 4.6.0 gave quite some crashes and now when it's a question of choice between 4.6.5 and 4.7.0, I wonder which is going to be more stable? Do I get it right that KDE has no 4.x.x stable release like 3.5.x was (at least seemed) in its maturity?

I've generally found that the latest versions are also the most stable versions of kde. While the pace of changes has definitely slowed since the early days, there are still significant stability and performance improvements between successive versions. There will probably be new bugs, but overall stability-wise, the new releases are typically more stable and less crashy.

Do I get it right that KDE has no 4.x.x stable release like 3.5.x was (at least seemed) in its maturity?

I'd have argued that even 3.5 had its fair share of bugs, but point taken; the 4.x series has yet to reach the levels of stability that 3.5.x had reached.

At the same time, I wouldn't compare the 4.x series too unfavorably to the 3.5 series. I find it a very productive environment to get things done in. If it means anything, given the option to switch back to 3.5 from 4.7, I'd most likely refuse.