I've always liked Biden. See here.
The most important factor for Obama, in selecting his running mate, should have been to ensure that the person a heartbeat away from the presidency is up to the job. Biden clearly qualifies. The second most important factor should have been to ensure Obama's election against an opponent who truly might be even worse than the worst president ever. On that, Biden was not the best choice. We can debate who would have been best, but there are at least a handful who would have been better. But Biden does bring one critically important strength to the campaign, and it is not his much-touted experience. Paradoxically, it is something for which he is also often criticized -- his oratorical skills. An already legendary speaker, himself, Obama must have taken particular notice.
Biden is a far superior choice over the other possibilities on Obama's short list: Evan Bayh, Tim Kaine, or even Texas Democratic congressman Chet Edwards. Biden brings Beltway experience, feistiness, and boatloads of foreign policy seasoning that Obama sorely lacks, and Biden knows more about sensible foreign policy than McCain, Tom Ridge, Tim Pawlenty, or Mitt Romney will ever know themselves. In fact, the Biden pick allows the Democrats to fight the GOP on turf that Obama heretofore has failed to grab and hold himself, which is the mark of a good pick.