From what I read a year or so ago from the article below, mechanical drives are the bottle neck and SSDs are, in my opinion, still to expensive per GB to make them feasible to use as an external drive.
It just doesn't seem like it's worth switching at this point until Thunderbolt becomes widely integrated on mainstream motherboards and until high density SSDs become as affordable as mechanical drive.
If that ever happens though, I'll be jumping ship. Those numbers are mighty good lookin'!