On paper the Villa Adriana seems a site that I would love -- a collection of architecture from around the known world in the 2nd century A.D. incorporated into a retreat (and eventual seat of power) for Roman emperor Hadrian in the hills east of Rome. In reality -- well, it didn't meet my expectations. Part of that could have been the difficulty I had in getting to the villa; no taxis were easily to be found anywhere in Tivoli on the weekday I visited last November, and I couldn't find a good schedule for the Number 4 bus. I eventually made it to the villa by taxi, and began exploring the grounds mid-day. I rather wish I had a guide, though, since, as others have noted, the signs are lacking, and what signs existed were written in an academic style rather challenging to comprehend. What I did enjoy were the variety of the ruins: the relecting pool around which was formerly a portico modeled after the Stoa Poikile in Athens; the beautiful evocation of the Nile at Canopus in Egypt; the tiled floors of the Vestibulum and Baths; the great views from the Rocca Bruna tower; the unique circular Maritime Theater that was mostly inaccessible due to construction. There is certainly a lot to see at Villa Adriana, and with a proper guide, I'm sure I would have appreciated it more. As it was, I thought it was pleasant, but rather forgettable.
Logistics: Villa Adriana can be reached by #4 bus from Tivoli or by taxi or private transportation; the grounds are walkable, but rather extensive.