For me, a small helmet-mounted mirrors works best. It's easy to find what you're looking for by just moving your head, which you normally do anyway, much like driving a car and monitoring your rearview mirror. Because it is mounted close to your eyes, and it's concave, your field of vision rearview is quite good.
Those who've never used a mirror will often say they don't need one, or that the radar is enough. Those who use a mirror, and are likely still using it, often say they couldn't imagine themselves riding without one. I'm of the latter camp. Yes, the dork factor is high for sure--it's hard to look cool with that little thing sticking out from your helmet. Being able to see behind me and what kind of traffic is coming up behind me is invaluable. Others may claim that this is of little value, but here are just some ways in which it is helpful to me.
1. When approaching an intersection I can see if a vehicle behind me is intending to make a right turn, and if it intends to pass me first to cut me off--the right hook.
2. When trying to get into left-turn lane to make left turn, I can monitor road ahead while find a gap behind to merge into left turn lane.
3. Seeing if vehicle behind moves over to the right when approaching me. If not, be prepared to wave, move right, bail right, etc.
4. Also useful in group rides to see if riders behind me are being dropped.
Radar is also useful, but when I commute I don't bother using my bike computer, or any other kind of head unit.