I reasearched them last year. From what I found, there are (in the interest of simplicity here, but you get my drift) "hard" ones and "soft" ones. Some user reviews I read reported that with the soft ones, they had to use garbage or other plastic bags as a first layer in order to keep things dry when it rained.
We wound up not getting one but on the drive home from Niagara Falls we had our luggage on the roof in duffel type suitcases and hit heavy rain so despite having washed all of our laundry before we left for home, everything in the suitcases got soaked and water stained.
After that, we decided to resume our search for a roof carrier, but decided the hard type would work best for us given that experience. Fortunately, soon after we got home I found a Sears hard type at a yard sale for $25 but we haven't used it yet.
Keep an eye out on Craigslist unless you want a new one. I think the hard ones run around $250-300 new.
http://www.sears.com/shc/s/p_10153_...21x00003a&ci_src=14110944&ci_sku=02872016000P
PS if you need it to lock, that could influence your decision as well.