Signal overlap is about the only "problem" with them, but they're designed to operate as a compliment to each other. We use dual band AP's here at work, in both our HQ and remote offices, and it's nice having the different networks to connect to. When I was in Anaheim last month, the "normal" band was fine for most things, especially distance. The 5GHz band was just as useful. If one gives you issues, you can switch to another (if you don't own the network), but if you have problems, changing channels often helps.
The nice thing about the routers that support the open firmware like Tomato or DD-WRT is that they have channel scanners to help you determine if there are other sources nearby that might interfere with a channel setting. It then suggests a channel to use based on surrounding interference. I'm pretty sure some OEM makers do this now, but don't quote me, I haven't used anything but my Linksys at home in a while (and the ones at work just keep working!)
