I'm by no means an expert on the subject but two years ago we replaced all our 180 and 250 drives with off the shelf 400s. Haven't had a problem since. I'm thinking of doing the same with 750s when I merge three buildings worth of storage into one. I currently have 5 Xserve raids with 750s and 3 raids with 400s. I'm looking forward to having 8 raids all loaded up with 750 drives.
Cheers
John Christie