I've used RTR several times and have had some successful rentals. The best way to do it is to visit a store and try on the dresses...and there really is no better place than Vegas to do this. However, having received dresses too big, too short, in poor condition, etc., I would advise that you never rely too heavily on receiving "the perfect dress." You should always have a backup in case you are disappointed with what you get.
In terms of customer service, I've had to reach out twice: once for a refund issue and once because the website didn't give me a return option. Both times my issues were resolved, so I've been pleased with the customer service.
My only recommendation would be that they offer a "do not like" option on the website. There are so many pages of dresses with more and more constantly being added, that it would be nice if we could filter out those that we've seen or tried on and know we don't like. I could easily cut out about half of the dresses that I know I would never wear, and it would just make the browsing process so much less painful!