I've used them several times and never had a problem.
That said, my girlfriend is a former Delta ticket counter/gate agent, and her advice has always been to avoid that site at all costs. Most airlines decide who will get "invol'd" (i.e. bumped) when the plane is oversold based, at least in part, on the price paid for the ticket and where you got the ticket (i.e. same price but bought direct from the airline is less likely to get bumped than someone who bought through another site). She said that the majority of people she bumped had tickets from Cheapoair. So, just something to keep in mind.
Like I said, I've never personally had a problem so that's just second-hand information. And booking this far out you should be okay - usually people who get bumped bought tickets closer to the last minute.