Sierra was the subcontractor for our homebuilder, William Lyons Homes. We contracted for a SEER 15 A/C and a HEPA air cleaner and paid dearly for it. Upon walkthrough, I discovered that the HEPA air filter hadn't been installed. Lyons checked and I was right; Lyons went to bat for us and forced Sierra to make it up to us by installing an electrostatic air cleaner (an upgrade) with two years' maintenance. After this, I checked to see if I really had a SEER 15 unit - I did not. Again, Lyons went to bat for us and forced Sierra to upgrade us to a SEER 17 unit.
Sierra was supposed to clean out the electrostatic air cleaner every six months for two years. They came but we didn't see much cleaning, but I attributed this to the fact that the filters are in the attic (unlike normal filters). However, my wife began to say that the aircon smelled bad. After the two years was up, Sierra wanted $500 to do the six-month service. Instead I called Infinity Air (see my review there) and their tech removed the filters from the electrostatic air cleaners and showed us that they had NEVER been cleaned before. He cleaned them and guess what, the house smells much better now, my wife is happy and the aircon's job is much easier now that it doesn't have to force the air through the clogged filters.
All I can say is that Sierra was completely dishonest with us in every single dealing. Use them at your own risk.