We go with ten second rounds in 4e, simply because we all agreed that the average combat shouldn't last approximately thirty seconds - it doesn't seem to sync up to the source material (in our case, movies like Conan the Barbarian or Lord of the Rings).
As we play it, the average combat is something like fifty to sixty seconds, which is still fast, but not quite as fast.
Were I to design a game from the ground up, I'd like to see it work around fifteen second rounds, which would be great, unless the time it took to process a round at the table was super fast - and then I'd prefer something like Shadowrun's super-fast three second rounds.