Overrated: Alf Soriano (despite his undeniably great start this year). He's worth about half what he's made during his career. Johnny Damon is another good one mentioned above. He was overpaid on his last contract, and is certanly overpaid now. Neither a great power guy or on-base guy. Solid, but not elite.
Underrated: For years it was Abreu, but I think his last contract got him his due attention.
Adam Dunn. For a LIKELY future Hall of Famer (IMO), the media and fans both focus a whole lot on his negatives. In some ways I blame rotisserie baseball. That whole batting average thing deflates his perceived value, while inflating a guy like Teixeira (who I also like, but is certainly no better and is probably worse).
I'll also throw out there that A-Rod - despite his salary - has been underrated by certain members of media, as well as fans, over the last couple years. In particular, anyone who thought the he was less deserving of the MVP than David Ortiz fails to account for the scarcity of offensive talent on the left side of major league infields.
Mark my words. If they had identical contracts, Theo Epstein and all of the other Sox brain trust would trade A-Rod for Ortiz any day of the week - this year, last year, next year, whenever.