My thoughts on Soriano have always been that he's one of the ultimate feast-or-famine guys. His approach to the game makes him the one of the most risky players to pick up. He's like a super-Jim Edmonds in that he can hit the crap out of the ball for a while and look like a demi-god, but when he crashes back to earth, it's not a pretty picture.
I've been around investing my entire life, and I'll tell prospective Soriano owners the same things I tell potential tech stock owners: Only buy if you don't mind the ups and the downs, because you'll get your fair share of both from them.
Remember that erratic investments are best judged over long periods of time. Using that standard, Soriano is a good investment. Just make sure you can handle the peaks and the valleys.