The versions I played (1.0001 to 1.0003) were remarkably bug-free. There was only one nasty one I hit -- CTD on Lake Yantar that recurrently happened a few minutes after entering the area. Something in the savegames, since it happened even between sessions.
I had finished the storyline quests there, so it was not a huge deal for me, but it would have broken the game had it happened before.
I just finished it, btw. Loved every minute of it (well, almost -- the very end of the next-to-last bit of the "true" ending crossed the line between "challenging" and "ridiculous", but I made it through with much save-and-reload, and it was only one bit in a very big, hugely immersive, and hugely enjoyable game).
I first "role-played" the ending, that is, I picked the one that felt right to me, and got the longer and I believe more satisfying ending. Then I tried the other "true" ending, and finally I tried the "false" ending. I would imagine that if you just zipped through the game and missed the branch leading to the "true" ending, you would get a pretty serious "WTF?" moment with the "false" ending.
The nice thing was that I only had to resort to spoilers once in the entire game -- I just couldn't find a switch I was supposed to flip. Turned out it was almost exactly where I was looking for it, I had just missed a (pretty obvious, really) side passage.
I played at "Veteran" level. It was challenging, but not insanely challenging. I really liked the way they balanced it out.
Notes:
I found a Vintar automatic sniper rifle pretty early in the game, in a stash at Lake Yantar. I made that my main gun for most of the rest of the game. The only problem was ammo management -- I had to go back to Yantar twice to stock up, since it doesn't grow on trees. (On the second occasion I had a quest to run too, so that was sort of OK).
Other than the ammo management (which is either a chore or part of the challenged, depending on how you look at it), it's IMO the best all-around gun for the game. The trick is that with it, you can outshoot the snipers that are pretty common in some areas, lots of fights are easy to do by sniping, and it's not completely useless close-up either, what with the fairly quick fire rate and ten-round clip.
I only switched to a combination of Tundar + Gauss gun once I got to Pripyat; then swapped the Tundar for that badass NATO gun with the integrated grenade launcher.