For jumping on the bike and riding 5-10 miles in an urban environment risers are great, they are comfortable, you're upright and don't need to be used to riding a lot.

For greater than that, risers start to suck for the very same reasons, you are too upright, you have one hand position and you can't stretch out.

At this point bullhorns or drops become a lot better. Bullhorns if you are never going to use the drops anyway or don't want to put on brakes or dummy hoods/levers. To me drops without some sort of hood/lever or hood/lever substitute aren't really that comfortable but on a fixed gear with 1 or 0 brakes, they look sorta well, dumb.