Main benefits of tubulars are that they are a lighter than clinchers as there is no wire or kevlar bead in the tire. A tubular rim can also be made lighter than its clincher counterpart as there is no "hook" needed to secure the tire. In its place, glue is used to secure a tubular tire to a tubular rim. Some people think that it is easier to mount a tubular than a clincher. Tubulars can also be inflated to greater pressures than clinchers.

Downsides mainly consist of tubulars being more expensive than clinchers. You have to spend money to get a decent grade tubular. Only place I have seen cheap tubulars of decent quality is from www.yellowjersey.org for $20 apiece. Availability can also be a problem.

One interesting product hails form Tufo: a tubular-clincher hybrid tire. Think of them as a tubular that can be mounted on a clincher. I would like to try out a pair, but the price is prohibitively high running at more than $50 apiece.

All in all, I believe that clinchers have matured to the point where the difference between the two is negligible for all but racing.