Phipps is one of my favorite places in Pittsburgh. I've gone at least once a year virtually my entire life. In that time, Phipps has greatly expanded and has so many more beautiful exhibits. As a kid I always pictured myself getting married there in the Broderie Room (And I probably would have if I still lived in Pittsburgh!) If you have kids don't worry about them being bored - Phipps offers scavenger hunt cards and there is an extensive outdoor area geared towards children.
Unfortunately we missed the Christmas lights this year, and I haven't seen them in quite some time. They often have special art exhibits throughout the year. I was lucky to see Chihuly's glass exhibit as well as the Frabel's Longfellows. A few of each pieces are now permanent, with Chihuly's throughout and the Longfellows in the beautiful Orchid Room.
If you live in the area, or expect to go twice a year or more as a family, you should definitely consider a membership. It's worth the price.