While my experience on Mavericks so far hasn't been as terribly unstable as some others in this thread, I do have to say that Mavericks isn't exactly the most pleasant OS to use. I've used Leopard, SL, Lion, and ML - I'd have to say SL and ML were pretty darn stable and pleasant to use. Lion was buggy in some ways, but wasn't that horrible.
I notice some weird system lags/sluggishness that seems to come and go on a random basis in Mavericks, as well as some glitchy behavior that seems unusual for a published OS - scrolling weirdness, stuttering graphics/text, general lag. Finder sometimes acts weird, pausing for no reason, or leaving window "residue" behind. Time Machine in particular is especially bad - the 'starfield' UI is very laggy, sometimes skips a time/day, and just is very choppy. Console forgets its window position/size upon exit. Those types of alpha/beta-level bugs that should've been sorted out by GM. I've reported all those bugs via bugreport.apple.com, with fixes 'on the way' without avail.
I'm not sure what it is about Mavericks, but it just doesn't feel "buttoned up" - it feels like everything is grudgingly working, but under the surface, there's a lot of badly written code/bugs laying around, and sometimes the OS keeps 'chugging along' to catch up. It's hard to explain, but it's a feeling/sense I get. It's not horrible, but it's not the best OS, ever. On a daily basis, I can go about doing my normal routine without issue, but I can just sense something isn't exactly right, if that makes any sense.
I think a general rule of thumb is that the odd-numbered versions of OS X has generally been glitchy/buggy (Panther, Leopard, Lion, and now Mavericks), while the even-numbered versions of OS X has been more polished and stable. (Tiger, SL, ML, and hopefully 10.10). I read somewhere that the same team of developers work on every other OS release - for example, the same team of developers who worked on 10.7 also worked on 10.9, and so forth. If that's true, then that would explain the "every-other" pattern.
I seriously hope Apple will commit to making 10.10 stable and buttoned up, and re-focus on the small details. That seems to have been forgotten with Mavericks (and to a smaller extent, ML and Lion). I suspect 10.9.2 was rushed out to patch the SSL bug, so hopefully 10.9.3 will tie the OS a bit more neatly together.