Paul isn't a reflexive contrarian--he doesn't oppose just to oppose. Rather, he has a core set of principles that guide him. They happen to be the same principles envisioned by the framers of the U.S. Constitution: limited government, federalism, free trade and commerce -- with a premium on peace.
When most members of Congress see a bill for the first time, they immediately judge the bill on its merits, or if you're more cynical, they determine what the political interests that support them will think of it, or how it might benefit their constituents.
For Paul, the vast majority of bills don't get that far. He first asks, "Does the Constitution authorize Congress to pass this law?" Most of the time, the answer to that question is "no." And so Paul votes accordingly.
That last paragraph pretty much says it all. If the majority of those in Congress followed that simple rule, we would live in a place that would be tempting to call paradise.
One question not asked in this is whether Paul's presence in the primary will have any effect in ending the preeminence of the Bush family and the neocons in the GOP. Paul is more diametrically opposed to everything the Bush clan stands for than any other candidate in the race. I too doubt he will capture the nomination since values, ideas, and principles always take a back seat to noisy, shiny crap in our society these days.