Rasmussen has conducted polling on Joe Biden at various times; I have included a recent poll where they took voters' temperatures on some of the potential VP contenders, as well as a poll toward the end of Biden's primary campaign, and then a baseline reading from 2006. In each case, Biden's favorables/unfavorables were pretty close to even:
VF = Very FavorableThese are not terrific ratings, and they get a little bit worse when you look at the depth of the sentiments, as Biden's strong unfavorables exceed his strong favorables by 5-7 points. Basically, I think he is identified enough with the (unpopular) institution of the Congress that he will be viewed by a lot of people as a partisan, but doesn't compensate for that by generating enthusiastic responses from the base, the way some other candidates might. Biden is fairly well-known -- by far the best know candidate of the Bayh/Biden/Kaine/Sebelius group -- so perceptions of him are liable to be fairly entrenched, and may not be enhanced by the fluffy sort of treatment that the VP candidate usually tends to get from the press.
SF = Somewhat Favorable
SU = Somewhat Unfavorable
VU = Very Unfavorable
Date VF + SF = Favorable SU + VU = Unfavorable
7/27/08 12 + 23 = 35 17 + 17 = 34
12/9/07 10 + 28 = 38 21 + 16 = 37
11/11/06 10 + 23 = 33 21 + 15 = 36
There are some positives, though. Biden's numbers are quite strong among seniors, a group with whom Obama is underperforming, and fairly strong among moderates. He would probably lock up Pennsylvania for Obama -- both because he is well-known in the Philly burbs and because Pennsylvania has an older electorate -- and might play well somewhere like Florida. (I don't think he'd perform as strongly in states like Colorado and Wisconsin, which are a bit younger and tend not to like the Washington establishment).
I understand that there is more to picking a VP than favorable/unfavorable ratings -- elections aren't won by out-nicing the other ticket. There is no doubt that Biden would perform well on the talk show circuit, and that he'd assuage the concerns of a certain number of older, foreign-policy-focused voters. That might be enough to make him a worthy choice. But I don't think he'd quite as appealing to the electorate as the conventional wisdom seems to hold.