Here's the gist of what I typed before it was rudely vaporized.
I have a dual-band router (Netgear N600). I also purchased a couple of dual-band client USB adapters Linksys AE2500 or something to that effect.
So the USB adapter works fine for a desktop, but having that crap sticking out the side of a laptop, netbook or tablet? Busted in 10 minutes. I hooked one up to my netbook and fried it within a couple of weeks because I'm a Netbook in bed guy. You wouldn't think it could get so hot from a USB port but it does.
So the reality is that you have all these devices that can't be upgraded to dual-band and enjoy very little if any benefit from the new-fangled dual-band router.
The other beef I have with routers is that they're terrible with the way they split up bandwidth between multiple devices. Instead of responsively reassigning bandwidth to the device that needs it, the router continues to reserve a major slice for a device that I'm not using.
If you live in an apartment building, it's actually rather rude to use the full 300Mbps capacity of the wireless N band, since you may well succeed in effectively shutting your neighbor down. There's so much happening in the 2.4GHz band nowadays, it's unreal. Your own cordless keyboards/mice/controllers etc can malfunction from being unable to get a packet in edgewise.
For these dual-band routers to be really useful, we need manufacturers of smartphones, tablets, laptops, netbook and such to build dual-band clients into them because adding the functionality with some sort of dongle just doesn't work.