One thing you should always do is aim for something better, keep writing ads and adding negative keywords until your CTR goes up. Test Test Test.
BTW, I'd say that the average CTR across all advertisers in the system is in the neighborhood of 2%.
In addition to CTR, don't forget to pay attention to your ROI (Return on Investment). There are tons of past threads on this forum that'll give you advice on this topic.
AWA
If it's real estate, 2% is ok, 4% is good and when things are really targeted 10%-15% isn't uncommon.
Brand keywords in a client campaign should pull 20% to 50% CTR most of the time.
Anything else, somewhere between 2% and 30% is possible. Never under estimate the value of getting really specific and writing really targeted copy. It's definitely easier to hit those niche keywords on Overture, get them cheap, and get great CTRs but with some work, you can rock on Google too!