Ramesh Ponnuru thinks it could be unusually important this year:
The race is a dead heat, according to the most recent polls, and the Democrats have been running as much against Ryan as against Romney. So the stakes are higher for Biden than they usually are in vice presidential debates. It’s safe to say that Republicans are looking forward to this one more than Democrats are.
Jon Ward argues that the effect is hard to predict:
How much a Biden win would impact actual voters would depend on how resoundingly he beat Ryan — which would in turn dictate the degree to which the media emphasized a possible shift in momentum — and on how many people tuned in. … A clear win for Ryan, on the other hand, would probably accelerate the current dynamic, with Romney gaining in the polls, Republicans growing more enthusiastic, Democrats becoming more anxious and media coverage portraying Romney as surging. If Ryan were to win, it would likely be on the basis of a policy-heavy presentation, which could amplify the impression among voters that he and Romney are a team defined by capability.
(Chart from The VEEP blog)