Realistically, I imagine Ryan will drive away some independents and win support of some Republicans that may not have voted or would've gone 3rd party. On one hand, it gives the Democrats another person to attack and rail against (yay, maybe we can get some more ad's tossing granny off a cliff) on the other hand I think Paul is MASSIVELY underrated by the die hard democrats/left leaning posters on this site in being able to explain and simplify Republican fiscal ideas that are going to be the heart of any chance they have to win. I think he does this far better than Romney and if used right could prove a benefit for right leaning moderates/independents that went with "hope" last election but are looking for an alternative this election.
I don't think it's the best pick politically, and I think it definitely has the potential to be a problematic one...but if I was going to lay money on it I'd probably go with the notion that it'll have little significant effect either way on this election. Yeah, it'll play into it and I won't be surprised if it hurts a bit...but I really think this one is going to largely come down to the top two.
I think the thing that makes this possible that it is okay for Romney.....Paul is not a huge vocal SOCIAL conservative. While the left and liberal on this forum and elsewhere like to proclaim any Republican that has conservative views regarding fiscal issues as "FAR RIGHT", I don't think the "far right" thing is going to work too well on the average person with Paul because for so many years the Democrats have tried to tie the notion of "Far right" to social issues and that's not something Paul focuses on a ton. This is going to be a fiscal and governmental election....that is, if either side ever begins to move away from just non-stop mud slinging, which will happen a bit as the actual debates begin...and I think Paul has a 50/50 chance in terms of his impact because of that depending on how well he does with his skill at explaining conservative fiscal views compared to how well the Obama campaign is able to launch attacks against those views.