Wow, that's a weird one and not what I was expecting. Some of you were thinking the same thing I was--that this is like the Quick220, trying to use the two phases of 120V to make 240V, but that is not quite what this is.
I read through the product description, and it's using two 5-15 outlets to combine their current output into an L5-30, which is still a 120V outlet type, but 30A. So I think this cable is tying together the two hots as one side, and the two neutrals as the other side instead of trying to span the two in series to double the voltage. So really, this might even work on GFCI outlets, because it is still passing current back out through the neutrals too. And since you're not trying to get double voltage, they wouldn't even have to be on opposite phases--just need to be a separate circuit, so you could get current from both.
So, um, I have not heard of someone trying one of these, but I would be kind of hesitant to.