A barrel is going to be a sanitation nightmare. That is to say, you'll never get it sanitized.
I'm going to be getting one really soon for my upcoming lambic project. Some of the bacteria that are in lambic thrive in wood, so I'm going to try to get a barrel infested with the desirable wild yeasts and bacteria. Also, I'll be letting it sit in the barrel for a year or more and I don't want to tie up my main fermenters.
An oak barrel flavor is supposedly integral to "real" IPA because the beer was shipped in barrels.
I live in wine country, so barrels are easy to come by. I'm going to use one that has already been used for wine many times. You probably don't want a new barrel, especially if it's American oak, because you'll get a heck of a lot of tannins and oaky flavor out of a new barrel.
All in all, it's certainly easier and more easily controlled to use oak chips, but a barrel is a project I'm going to be starting for some specialty wild yeast beers. Cheers!