I would recommend Kaplan. Ok, before all of you beat me down for recommending 'Krap'-lan, hear me out! Yes, the tests are brutal. Unrealistically brutal. Yes, the tests will probably knock your confidence even lower than you thought it could go. But,
they are still great practice. Given that, they are not for the fainthearted. If you can handle the stress of continuously scoring between 550 - 650 on the tests, take 'em. On the other hand, if your confidence is already at a low, I would recommend not taking them
now. But I would still recommend taking them after some preparation. Btw, I scored below 600 twice on Kaplan CATs and ended up with a 700+ on the actual GMAT. So don't stress too much about the actual scores

Just use them to practice. On a final note,
always do the AWA sections on the practice tests. I cannot stress this any more. That extra one hour makes a huge difference.