Simply one of the best in the nation!
First off, I am from Pittsburgh and hold a family membership to Phipps, so I will reveal my bias right there.
However, even if I did not have homer bias, I think I would rank Phipps right up there with the best anyway. As proof of that, I would point you to all the other reviews on here.
To add to what has been said and maybe repeat what has already been said, Phipps is just an amazing place. It is centered perfectly Schenly Park, next to the acadamia region of Pittsburgh, Oakland, home most of or at least portions of all the major world class universities of Pittsburgh. So Phipps gets much support from the universities directly and indirectly, through not neccessarily money, but intelligent and talented individuals. I imagine that is the biggest reason it has stayed one of the finesst and oldest conservatories in the US.
Phipps is just completing a major renovation. We were after the new visitors center opened this past Christmas, but before the new tropical room was open and ready. We went to an evening program that was under the "Enchanted Forest" (I think that's what it was called) billing, which had the walkways lit by candlelight and a polish jazz/polka clarinetist and accompanying guitar player perfoming. There were many other performances scheduled nightly over a several week period spanning the holidays. It was wonderful and surreal. They had a circus type theme trains set up in a center room, and many of the rooms in full Christmas and holidal glory, and at night the lights were all wonderful.
Phipps offers a butterfly garden in the spring and summer, and we will be going up that soon as well, and we have to see the new tropical room that I think is completed now.
We have been to the orchid shows the last few springs, but missed it this year. Phipps has a wonderful collections of just about anything you like, succulents, ferns, orchids, tropical plants, etc.
I have been to many conservatories and botanical gardens, Krohn Conservatory in Cincinnati, the Butterfly Conservatory in Niagara Falls, the Elizibethan Gardens of Manteo Island, the Queen Elizabeth II botanical garden of Grand Cayman, Missouri Botanical Gardens, Brooklyn Botanical Garden, Toronto Botanical Garden, the Botanical Gardens in Montreal (Jardin Botanique de Montreal), the Niagara Parks botanical gardens and floral clock, and many others, and can confidently say that Phipps ranks among the finest in the world.
Right now Phipps is featuring Mythical Beasts, with a 70 foot dragon, and statutes and flowering and vineing creatures from the worlds myths. This and the butterfly forest will be available until October. The Mythical Beasts show is part of the Pittsburgh Regions "Pittsburgh Roars" program. There are some upcoming jazz in the garden shows too -- those are wonderful. Check their online calendar for dates and times.
Phipps is one of the "goods" that you simply must look into to experience what Pittsburgh has to offer.
If you have more time, be sure to take a stroll around Schenley Park as well! It's an amazing area.
Enjoy!