ByrdMan, the "checks" I've received in junk mail were not "cashable." You couldn't walk into a bank and deposit them in your account--they say "non negotiable" on them. The funds could be used, however, for merchandise from that merchant. To tell you the truth, I've paid very little attention to them and threw them away without studying how they worked, mainly because (1) I was annoyed at the sleazy tactic of making me think I had a "check" and (2) I wasn't interested in anything they were selling (cheap junk).
The links I give above do give more info. The one at indigorising says it reports to two CBs. According to the first link, they do report the total size of the "check" as a CL. Before I made a "down payment," though, I'd like to hear from anyone who might have used one to get a little feedback. I'm in more than enough debt for now, lol, unless it would pay off in a higher CS.
I think you can use a fraction of it (see the ezine article above).
I guess I'm just going to wait and see if I get one of these in a junk mail offer and then investigate it further. I don't really trust the one in the second link I mentioned--I have enough credit/debt problems and don't need to actively stir up anything else.