Flushmounts look great on the bike, but have very poor visibility on the street. Even if they are bright from the side, you can't see them from the front. Think turning left at an intersection against oncoming traffic (or right, depending where you live), which is one of the most common motorcycle collision scenarios. In my opinion, the Zero-11s have even better visibility than the OEM signals cause you can see them from all angles. They're not cheap, but I think they're worth the price.
Or maybe I just really really like Rizoma. Ha