Our Aussie buddy, 4seventy, is a regular user of O/U rifles. Maybe he will add his experience to what has been said.

Although I prefer the style of the SxS, I would not hesitate to use a good O/U in the field. In fact, I used to own a setup of Valmet 312 that included 12/12, 12/.222, and .30-06/.30-06. I used the .30-06 barrels extensively. They were superbly accurate and accounted for a lot of downed game here in Western Virginia. I had a 4x Weaver mounted. Accurate shots to 300 yards were routine.

The O/U enjoys one advantage that is undeniable: cost! A good O/U can be had for a lot less than a comparable SxS. Here's mine:

Curl