Came here on vacation when I was staying in St. Pete Beach. This place is definately worth a trip for four reasons.
First, the museum itself has an outstanding collection. They have a few pieces from ancient Greece, a few from ancient Rome, quite a lot of European from fourteenth century to present. Mostly paintings but a few furniture pieces, silver, watches, etc. You are allowed to take pictures without the flash. The price of admission paid for the museum alone in my opinion. There is also a nice courtyard with statues.
Second, the grounds have a number of large banyan trees. This was totally unexpected and very impressive, quite a treat to see.
Third, there is a beautifully appointed Venetian Gothic house. They call it a mansion, but it didn't seem that large to me. I declined to pay extra to go upstairs. Very much worth the walk (or the tram). For people with trouble walking, they have a very extensive tram system covering the whole grounds.
Fourth, the circus museum was neat. They have a bunch of circus artifacts like a cage car, carillon, and costumes. They also have a separate building with a huge scale model of the entire circus. There is lots of supporting material like posters, videos, and news clippings. I was never really that into the circus, but I thought this was worth a trip as well.
On the whole, this place was great. Well worth the drive from Tampa and the price of admission. The place was practically deserted when I was there.