I've called upon Mast Air twice in the past year.  The first time was because my furnace wasn't working (55 degrees in the house!) and the second time was because my AC was emitting a sporadic burning smell.  Both times, they were extremely fast.  They are very well priced, and even offer a great yearly maintenance package that includes two visits and a discount on repairs.  Mike was very nice, pleasant, and did excellent work.  He even let my boyfriend come up and see what the problem was to visualize the situation (apparently, no one had EVER cleaned the 12 year old AC, ick!).  My air is now colder and the AC on less, after just one simple visit!  They came recommended by the lady that sold me her house, and I am so glad she gave me a referral.