Monster Park in Bomarzo

I would say that it depends on how much time you have and how often you've visited Italy before. In other words, it's not a top tier sight. However, if you're exploring the Roman countryside, it could be part of a nice day that also included some combination of Villa Lante (in Bagnaia), Viterbo, and Tuscania, but not all of them. Two hours or less for Bomarzo.

We took our kids there a couple years ago and had a great time. It was such an unexpected surprise to find this place in the middle of the woods. It wasn't crowded at all in early October. I've only seen the park mentioned in one US guidebook, so I don't think you'll see many American tourists there. I don't know that I'd make the park a destination, but it was a fun diversion to break up the drive between Orvieto and Naples. I'd say 2 hours, max, would give you plenty of time to see it. If I remember correctly it's only about a 10-15 minute drive from the Autostrada.

I was in the Bomarzo Park ("Parco dei Mostri" or "Sacro Bosco") less than three weeks ago, on a Saturday morning. There were lots of Italian school kids as well as individual families with children. It's a historical curiosity, an "artistic joke in stone" worth a quick visit (no more than 90 minutes) if you've got nothing better to do. You can also see Villa Lante in Bagnaia on the same day.

If you don't have a car, the Bomarzo Park is really a bit tough to reach on foot from the bus stop in Bomarzo Town, although I myself reached it in about 20 minutes. It's a very steep walk downhill to the Park entrance, and another very steep walk uphill on the return. There are several COTRAL buses from Bomarzo to and from Viterbo from Monday through Saturday (30 minutes, 1.20 Euros each way). I honestly don't think the effort is worthwhile, so I would suggest you skip the Park unless you have a car.

The Park is featured in a black-and-white movie from the early 1960s with a very young Donald Sutherland!
The rock carvings have not been "restored" since they were carved 500 years ago (apart from removing moss which easily grows on the carvings due to the very humid conditions). A walk in the Park today is likely no different than how it was in the Sixteenth Century (apart from electrical lighting and plumbing). The area around Bomarzo is heavily wooded/forested, and the vegetation is lush. Bomarzo Town is quite austere and atmospheric; very, very different from the Tuscan hill towns not far away.

By contrast, Villa Lante in Bagnaia is very well connected in 20 minutes by hourly city bus no. 6 to Viterbo (1 Euro each way), and is a very easy 4-minute walk from the bus stop in central Bagnaia. It's also delightful to visit, and won't take more than 30 minutes. I highly recommend it.

And don't forget to visit Viterbo!!!!!

Thanks to all of you for your input!