I just had an audit done for $75 this morning. I have to say for me it was marginally worth it, but that is only because the house is in good shape already, I would have benefited more had I needed some sealing/door seal service done. My door blower test revealed a tad bit over sealed, and they recommended to do no more so the house can breathe. Other than replace some faucets with low flow aerators and added dimmable recessed can light bulbs I didn't get too much for the service. They also tested my furnace ducts for leakage and reported that they were in good shape for a 4ton AC system. I have to say the dimmable CFL's are pretty good though, I tried them once and thought they sucked, but I guess the trick is to let them warm up un dimmed, then dim them down. The phillips brands the guys installed are pretty good. On the plus side I feel better about my windows etc, and am not going to worry too much about trying to do more sealing etc, I think I will focus my efforts this spring/summer on outdoor bulk coal storage.
So in conclusion to decide if the service is worth it, do you need CFL's, could your house benefit from new door weather stripping, do you want to have your ducts checked etc.? If you know your pretty air tight already don't bother, but if you have some drafts its pretty worthwhile.