Belts are still fairly new and uncommon so frame compatibility and cog/cassette availability will improve in the future. They're not likely to ever over take chains, however. My belt drive commuter bike is quieter and smoother than any chain driven bike I've ever owned. Belts are cleaner than chains which can be an issue for commuters who bring their bikes onto trains or into offices. Probably the biggest issue for belts will be cost, not just of the belt itself but of the required multispeed hubs to get sufficient gear range. Those hubs, it should be noted, are more than heavy enough to wipe out the weight advantage a belt has over a chain.