Now that they have a sale, I am thinking to get Witcher I. Straight to the point, is the game buggy and likely to crash a lot, performance is poor, etc?

No. At release there were complaints about it, although I found it worked just fine on my system. The only issue I had was the long loading times. But that and many other reported issues have been addressed in patches. The game should be absolutely fine now.

No bugs to speak of, although there are sometimes a few issues that need resolving. Check the sticky thread for a list of common fixes if you have any problems. Almost all can be fixed easily.

I encountered a couple through my early playthroughs. Can't remember which version, but I got stuck in a cow in Chapter 4. My most recent play through was pretty much perfect

The only thing you might need to do is jiggle the lightning settings. Some cards get some strange things happening there, but it is far from game breaking.

Yes there are a couple of bugs.
I ran into two:

A Gravediggers Gratitude.

You can get this quest AFTER the autopsy but he doesn't tell what he needs for you to enter the cemetery. The dialog just ends abruptly. You get a quest tough. But you'll not be able to finish it. Neither Vincent or Thaler have any dialog options for you to get a pass into the cemetery.

I forgot the name of the quest in Chapter one, but it involves Coleman.

Coleman has been killed in my playthrough. And the quest did not reflect this change. All I got was: "Go see coleman in the Hairy Bear Inn"

Yeah right... I'll need a shaman for that now, won't I? Talking to the dead and all that.

I had a couple of other issues that I can't remember right now but they're there. Mainly the load times (saving actually) who are a pain. They are not THAT long but they are CONSTANT. Especially in the city where you need to enter/exit a lot of buildings and all of them have a load bar waiting for you.

So, the game (so far for me) the game IS NOT buggy. But it has a couple of bugs.
It doesn't make it less enjoyable though. I love it all the same.

So much in fact that I'll buy Witcher 2 here on GOG (have Witcher 1 on Steam).

I only ever saw a few bugs, mostly things like the journal not updating properly which I find annoying but can roll with it.

As for performance I remember the odd kind of skip but nothing that ever really impeded gameplay or made want to kill something.

I have run into a few graphical bugs, but nothing that was game stopping. The only thing I would recommend is turn off Anti-Alising if you are running a AMD/ATI Graphics card. I am running a 6990 2gig, and the ran horribly with AA on. This game is awesome and you wont be disappointed.

Every so often, if I try saving a game, it crashes. Sometimes it happens when I save too often, other times too quickly, but otherwise it just happens. Keep that in mind before getting this.