On my motorbike trip of the Mao Hong Son loop I stopped at Pai immigration to extend my visa. I was there about 10:30am, the girl said I was the 2nd person there this morning.
Quick and easy, there were surprisingly 5 officers there. Compared to Bangkok, everyone was so friendly and actually engaged in a little conversation. I was out in about 15 mintues with my extension.
I've never been to Chiang mai immigration, but I've read the ques are similar to Bangkok. For extensions, I'd ride 3 hours to Pai immigration and do this everytime.