Purple Martins are really super birds in my opinion. They like their homes built very high... 18' - 20' up is ideal. The opening is also critical. I believe it's to be 1 5/8" in diameter. Might check me on that since it's be a while since I've built one for them.
Martins are very clean birds as well. They clean their house out before they leave each year. And if sparrows or whatever has moved into their house, they won't re-enter it. They simply don't like nasty.
Martins will definitely run crows off as well. I've seen them gang up on crows that were trying to invade and they'd beat the crow up and usher it off the premises. They're most interesting to me, as you can tell.
Like others have said, they send scouts ahead about February. If they find suitable housing, they'll leave for a couple of weeks, then bring back the ladies. Martins will clear your area of mosquitoes - that's for sure!
I hope you'll check the specs on their housing and put up some nice fixture for them. They're very worthwhile.
