I've been to a few plays at the Olney Theatre, and I enjoyed them quite a bit. I'd recommend any of their productions quite highly. However, I've noticed a trend, and while it may not always hold true, it has happened too often to be a coincidence.
The leads in the plays I've seen were good, but definitely far from great. They all had a supporting actor (or more than one) who really carried the play. It would seem the "special guests" they sometimes get to star in some productions are really not as good as their regular troupe. Unfortunate, but it shouldn't prevent you from attending a performance.