I think Rand Paul's a high-risk, high-reward candidate.
He might be able to excite people who don't usually vote. That could give him a boost over a generic Republican.
Or he could end up scaring voters in the middle with proposed budget cuts and decriminalizations, sending them running to Joe Biden, a man whose decades of public service suggest significant reliability.
Paul might also be able to run as a generic Republican, albeit much less hawkish, turning the election into a referendum on Obama and Biden, with the results largely dependent on the state of the economy.
Totally agree with this assessment. Both are high risk, in a sense, but Paul is not going to be well-received at the national level if he runs as a single-issue candidate. The deficit is going to be lower by 2016, and holding the opinions that you should get rid of programs like NOAA is going get 30% really excited but scare the other 70%.