Not sure that I'd ever actually use it for a phone/tablet, but it might prove useful for when I want to lay down and still be able to chat, change a movie, etc on my desktop PC. Not a bad price if it's decent quality, especially noting that it includes the receiver.
If I have a tablet, it's in my hand. How can I hold a tablet and this device at the same time? If my smartphone/tablet is in a dock, I'm usually around something else with a larger screen on which to watch videos. I don't take my dock with me when I'm at my friends' houses.
Yargnit was right, this would be better for a HTPC or something, not a smartphone or tablet.
I've had one of these for a while now with my HTPC... it's amazing, UH-MAY-ZING. That being said, I'm with the others when they say that this wouldn't be the best tablet keyboard. The advantage of a physical keyboard for a tablet, I thought, was the ease of typing. That advantage would be lost if the keyboard has seriously compromised comfort for size. Ah well, get one for your HTPC. DO IT.