I used it about a month ago... I might have had some connection problems, because the first time I submitted my request, I got no automatic reply. A few days later I emailed MS, asking for a confirmation... They sent a note back that they never received my reservation request. I decided to give it another chance, and went ahead and tried a second time, about a week after my first try. This time I did get a confirmation email, and a couple days later I got confirmation that my reservation was made...exactly as I had requested.
Bottom-line, the fact that you do not get real-time feedback that MS has processed your request adds some risk. However, if you don't have time through the week to call, and you're not making a ressie for a popular time of year, the on-line ressie form is a satisfactory alternative.