Yep. I've experienced both very short and rather long waits. I'd plan on at least an hour.

If you're going on to other cities, you might want to make sure you can change your reservations if you miss the train. Otherwise you're at the mercy of United being on time.

If you're going into town, the express train leaves every 1/2 hour and costs 9.50 Euro. I've also used a car service that charges 35 Euro from the airport to anywhere in town. http://www.romeshuttlelimousine.com/

I think you have to book in advance, though.