its hard to say exactly what is good CTR , 10% is better than 5% as all knows , The CTR depends on types of site and on many other factors , 5% CTR is very good CTR for forums , But 10% is not good for certain type of sites , And amount of traffic should also be considered on judgment , like if you get 20% CTR with 100 impressions per day , Its not good to say but if you get 10K impression per day and have 5% CTR its better than that of 20% in former case.
coz those visitors are used to the ads and no more interested in those. Got it? Search engine visitors clicks the most.
Average for most people is something around 2% or less. Anyone who knows how to place ads well, should be doing 3.5%+ at least. 5% in any market is great.