Skytest have been around for some years and they definitely have made a name for themselves in their field of business. I once bought their software and I feel it might have helped me, although, as PigeonVoyageur mentioned, I might have passed the test anyway. While I can't tell whether their software is worth 70 EUR, I can at least say they won't rip you off completely. So, as long as you're the type who doesn't mind paying for something in advance, I wouldn't worry about giving them your credit card number.
Should you, however, plan to prepare for the DLR test, I would advise you to keep in mind DLR will provide you with free practice tests. I found these more than sufficient.