This company could be seen as something of a scam. It's to be paid money for something that's individual and subjective; to try and turn people who don't believe they'd get in into what it believes (and it is unconnected with oxbridge admissions in the same way as, say, the Access schemes are as they are part of the universities and it isn't) will fit the mould of what admissions tutors are looking for.
In short, there's no real requirement that they have that much success. You cna hardly sue them for breach of contract or whatever if you don't get in; as Niccolo says, you're not paying for a golden ticket that guarentees entry, you're paying for advice which can't guarentee it; they can state that it makes applications more likely to be successful, but that's not something you can rely on or that guarentees anything.
I think to sum up the opinions of the Cambridge people above, interview preparation can be useful, and help with the application process makes it a lot easier- though it isn't vital. However, the opinion on here is that Oxbridge Applications is a waste of money, possibly dodgy, and is preying off the fears and expectations of people. Not so much a problem with what they do, as the way they do it and the fact that they're doing it commercially.