Well — a bit of perspective on this — the most likely pair of USSC justices to go in the next 4-8 years are Stevens and Ginsburg. Not much further left you can go beyond them. The conservative side is aging (Thomas and Scalia in the early 70s) somewhat, but not nearly as frail. At worst, for those two anyway, the future picks by Obama would be a wash.
plus David Midnight jogger souter and Anthony half-stepping Kennedy. as for Kennedy you will replace a justice who is right 5% of the time to one who will be consistently communist.
Yes, they will be a wash for now. Unfortunately, they may be on the court for decades.