Giambi has been productive but injured. Sheffield was also productive. Johnson was also injured, but he did have a bunch of wins. A-Rod has been MVP. Out of all of them, I don't know how you can say Mussina hasn't worked out - since acquired, he has been absolutely fantastic, carrying the rotation at times.
Giambi wasn't given a hundred million dollars merely to be productive. He was a league MVP in 1999 and drove in 120 runs with a .342 avg. for a small market team not known for offense, and signed by a team that had just won three consecutive championships that was expecting more. And since then, the Yankees haven't won a championship and Giambi hasn't sniffed an MVP. In fact midway through his contract, as Giambi hadn't sniffed .300 since his first year in the Bronx, the very guy who signed him amid all types of hype was looking for technicalities to jettison him from NY.
Sure, ARod has been an MVP. He was MVP before he came to NY. He wasn't brought in to help them get to the playoffs (or bed strippers and aging pop stars); the Yankees were more than capable of getting to the post season before he arrived. Sure, he should win MVPs in NY, surrounded by a loaded offense. What didn't work out for the Yankees is the fact that he'd be a deer in the headlights during the postseason (Hey, Scott Brosius won a WS MVP).
Mussina has had a nice career in NY, but please, re-read my post. I never said he didn't work out; I said the big dollar signings didn't work out the way they had planned. I didn't even mention Hideki Irabu, Kei Igawa and some others. My point is that this decade there has always been hype and spectacle about all of these signings and it hasn't been nearly as successful as the days of far less heralded players, like Paul O'Neill, Tino Martinez, Bernie Williams and Jim Leyritz.
