Pros (+) We chose Parco die Principi primarily for the location, which is right next to Borghese Park and close to some other principal sights in Rome (Trevi Fountain, Spanish Steps, etc., which were by the way mobbed, even in October.) The hotel is beautiful, the rooms very luxurious. We did not use any of the facilities.
Cons (-) For such a high-end hotel, we thought breakfast was very sparse, poor selections, nothing you wouldn't see at a hotel half the price. We were also not happy with the concierge service; they could not offer any advice on tours other than a single company, and were no help at all in arranging transportation to Civitavecchia, which I finally arranged on my own.