When I first picked it up it was EXTREMELY difficult to make any use of the thing, then I read the Manring/Lawson forum and realized that you have to practice it as if it were a technique in itself.
You don't just begin to slap (er, and make it sound not like crap), you need to practice. The same thing applies here. I've been messing with it for about a year now and am actually able to sustain notes.
Somthing to keep in mind though, IME, the fatter the strings are the longer they take to start sustaining. Dunno if you care about that.