How accurate is Google Analytics?

I get really weird traffic when I look at Google Analytics when it comes to referrals from other sites. I'm a web designer and put my link at the bottom of each site I make with my client's permission. I've made about 10 sites so far. I'll not receive any hits from a particular site and then all of a sudden I'll get 1 click over every day for a week from that site. I check the site, and the site's traffic had remained the same on average. Then from another site, I'll receive nothing for a month, then all of a sudden one day I'll receive 3 hits from that site on the same day. This is weird... How accurate is this?

In my experience, it's pretty accurate. It does vary a bit when compared to other analytic tools, mainly due to the bots it "hides" from results.

It's pretty accurate but I think it can only register users that have javascript enabled? Something like that I don't remember.

AW Stats from your server is going to be the most accurate, but Google Analytics is probably 97-99% accurate.