I've put about 2 hours into the game so far. Brutally hard. If you love realistic difficulty, you will love this game. Very hard.
I have only had one bug so far, surprisingly a bluescreen. Otherwise the game is good. They put you through a couple linear "stages" at the beginning, but then thankfully the game opens up into it's usual open-ended style. It seems to expand on this in some ways by having each area (as far as I can tell) have it's own dynamic set of events going when you enter them. That has good and bad points, but too much to go into here.
The only "negative" impression is the performance. I'm running a 7800GTX card, and the game is shit on my system. I have never seen such bad performance from a game, but I don't feel it's the game's fault. You have to figure it features fully dynamic HDRI, so... go figure. I will see how it goes when I get a new card. Otherwise it runs fantastic when the render engine is set to "All Static Shadows" (or whatever the lowest rendering setting is), but at "Maximum Detail".
For Overon - I downloaded the 1.0004 patch for my retail version tonight. On the same page was another patch for the "online" version (or something similar), so I can only assume they released a patch for both boxed and online distro's at the same time.
I really like the Stalker games because I understand what the developers are trying to make. I typically pirate every game before buying to see what it will be like, but I generally purchase the Stalker games right away just to support these guys. That's rare for me, but knowing that might make a difference for you.