I am about to pick up a D-Link DIR-825 which can do simultaneous 2.4ghz and 5ghz with its two radios. Gonna do a direct comparison to my Airport Extreme/Airport Express combo that I'm using now to run the same (Extreme-5ghz, Express 2.4ghz). As rock solid as my current combo has been, due to the location of where I need the express in relation to both my home stereo & my airport extreme, the combo proves to be an annoyance to my wife as I need a long ethernet cable running between the two. I decided I would rather get a dual band simultaneous router to handle both, and just use the express for streaming audio.
I'll let you know how it goes.
One thing I might add though, 5ghz has been a GODSEND for me. 2.4ghz is VERY crowded in my development, I can pick up almost 20 different access points on my second story. As a result 2.4ghz does not work very well anywhere but my ground floor. 5ghz is pristine and empty, so even though it can't travel as far, the speed and reliability for me has been a huge improvement.