Polls (scientific ones, anyway) are usually reliable enough to give you a ballpark estimate on where everyone stands...
But not this year. They try to sample "likely voters" and they look at old registration rolls and the like to find them, but some candidates this year are attracting a lot of people who have never voted or participated in the political process in any way. Obama on the left and Paul on the right are both good examples of this. As a result, the polls are less meaningful than usual this election season.
I bet we are in for some surprises. It's shaping up to be a weird race.