the short and sweet story. "get a gen 4"
now for the long story....
Glock had a recall on the recoil spring assembly with the first production run models. If you find a deal on a used one just call glock and if yours is on the list they will mail you the newer revised recoil spring assembly.
here's the link to glock explaining this.
http://us.glock.com/customer-service/re ... g-exchange
If you get a new one they've been fixed for a while so you shouldn't see any problems.
After handling the gen 4 I traded my gen3 17 and 26 immediately for the gen 4 models. Got the 17 used and it was on the recall list. Glock had the revised recoil spring assembly in my mail box within a week. Keep in mind I had no problems with the original.
Also there are visible design differences you can look for in the recoil spring assembly before you buy. heres a youtube video
http://www.youtube.com/watch?v=b_ojw6LDibs
Now if I were to stumble over a deal on a used gen 3 I wouldn't hesitate to scoop it up, but the gen 4 changes to me worth another $75-$100 over the gen 3.