You need Operation Arrowhead in addition then to be able to play DayZ (which is frecking amazing even with the bugs and typical OFP/ARMA clunkyness)
Bugs wise, DayZ is Alpha so you know what to expect.
Arma II - worst bug is dying when crawling over things like rocks or tree branches. Doesn't seem to be fixed at all, or maybe just some of the missions/islands still have it badly.
In DayZ I haven't broken any bones or died because of it, but I've heard reports.
Had a funny server today - people were talking about picking up strangers with a chopper they fixed/found.
According to the chat, nobody was actually backstabbed and people had fun. This is something a lot of people report on not existing anymore for some reason. I was in an UK server, so could be just that the people there weren't twats?
But as a whole, I don't have any compaints about how it plays in Alpha, most problematic currently is that you shouldn't leave your main rifle in the backpack when you log out, but the last time it didn't dissapear for me, so it's 50/50, the sidearm has survived several logons already though.
Other than that, there's a few clitches here and there as one could expect, but at the alpha state this little mod already outperforms (I think most) games I've played in the recent years when it comes to immersion and heart rate.
Massively tense moments, encounters with Zeds and other players (friendly and hostile)
Oh and I haven't died since the update which removed pistol as the starting weapon, so I'm not sure how bad it is to survive your first spawn without one, but one should be careful I suppose not to run into big cities and get picked off by bandits.
It's interesting how well this mod simulates what would most likely happen in real life.