I've got a nice dual band Linksys router at my office. I've got it set up to deliver 802.11g service on the 2.4GHz band and 802.11n on the 5.8GHz band. I'm using dd-wrt with it.
Since I'm the only person in my office who has an 802.11n NIC, this is all just fine.
I don't notice the difference in speed, particularly, but I seldom want to do anything more than connect to the internet and print from my laptop. When I want speed I plug in to a gigabit port.
What I do notice is the fact that I have a 5.8GHz signal that has a slightly larger radius and slightly higher strength than the G-only service that used to be present. I can blanket my whole office with both 2.4 and 5.8GHz signals from one AP, and that's excellent.