There was a bit of rumour surrounding Us rentacar a few years back, as one of the directors used to work for a company that DID go bust. There was some concern that Us rentacar might go (ahem) "mammary glands up" as well. The owner of the company actually contacted a few Disney boards reassuring people that they were legit (and informing us that it was his son who had worked for the company that had gone bust, but had no financial stake in it).
Now, that was several years back (2004 I believe) and since then I know loads of people that have used them without a problem. Personally, I did book a car with them but had to cancel it - my queries were dealt with very quickly and the money refunded within 24 hours.
They do seem to have built a pretty good reputation (and also are a sponsor of another Disney site), but as there are no regulators for car hire companies, then its impossible to 100% guarantee your booking is safe (but then that is true of most of the smaller car hire companies). As the previous poster says, the only safe way to book is through someone like Virgin who have an ABTA number...but then Virgin quoted me £400 more than US rentacar so you have to weigh up the risks.
Us rentacar take a small deposit and the balance 8 weeks before departure. If you put it on your credit card you will have some "insurance" if things go wrong.
