The major problem with them is the type of holds on offer. You'll want to be doing one armers on the jug at the top, but grab this and it pulls the whole thing sideways and the hold position ends up shite. The next hold down causes blisters, the one below this does as well, and the bottom hold is too small to really get worthwhile one arm training on (risks injuries), and too big for two armed work. All in all it's a very poor substitute for a finger board.
It's not even very portable, you need a beam or hooks with plenty of swing space, or a chin up bar, in which case you may as well just use the bar and get a better work out. They're heavy as well.
You can tell I really like them, go buy a set :D .