After reading through this thread and seeing a few other glowing recommendations, I bit the bullet and bought ARMA II and installed the game. So far I've been having fun, but I've also been thoroughly frustrated a few times.
The most frustrating time was when I was had quite a good haul, had full health, and was trekking up to the airfield. Right around the time I got there, I guess the game went into sunset or something, but every time I looked west, I got terrible glitching on the graphics. Flushing or changing settings didn't fix it -- I have a 6870 with the latest drivers, so I'm not sure what the issue is there. Next thing I know, I'm being attacked by 3 Zs. I dispatch them without drawing more attention to myself, but I have a broken leg. No big deal, I have a bunch of morphine in my backpack. I open up my backpack, and there are 5 tents...which is odd because I had only come across one tent in my travels to that point and definitely didn't pick up 5. No morphine, so now I'm near the airfield with a broken leg and no way to mend it. Got about 500m before I got spotted by a Z who brought about 15 friends with him.
My next frustration is with Z aggro. At times I'll be crawling right next to a Z and he won't aggro -- other times I will be crouch walking with no Zs in sight and all of a sudden a Z will come running around a corner about 100m (already aggroed without LOS, since he's running). Sometimes my character will stand up on his own when crossing a threshold, which often results in aggro as well. The ear and eye icons never seem to correspond to what I'm doing (e.g. sometimes crawling in the middle of the street is no eyes and no ears, sometimes it's two each), and the levels don't seem to correspond much to whether a Z aggroes or not.
The other big frustration is starting with no weapons. Z concentration and aggro issues dictates that I need to crawl from about 300m away from a lootable location, which takes forever (and often I still end up aggroing), and I've only found guns in two lives so far, so I can't run into a location and trust a weapon will be there. Couldn't we start with a crowbar? It's a fairly useless weapon but at least gives me the option of running into a building and beating a random Z that has aggroed...
Another frustration is that the advertised GMT doesn't seem to correspond to the time the server actually has. I was trying to play during the day yesterday and tried about 10 GMT-4 to GMT-8 servers before I found one that was actually daytime -- similarly, at night I was joining GMT+8 ones and they were also night (should have been around 10AM). Are people advertising their servers incorrectly, or does the time drift?
I also think the game world is way to big to have only 40 or 50 maximum. I've been playing on mostly full servers (that's the best way to figure out if a server is currently in daytime, as they usually are the most populated), put in at least 12 hours of playtime, and spent most of my time in and around the major cities, and I have only seen 3 alive players, and they were all freshly spawned (or at least appeared that way in the half a second I saw them before they ran off). I've come across a number of corpses as well, but with one exception, those have all been empty except for starting equipment as well.
Lastly, I have yet to come across a single vehicle. The map I am using indicates vehicle spawns -- I have looked at probably 50 of those spawns on different servers and still haven't seen one.