I booked a hotel in Paris a couple of months in advance with Hotels.com. They confirmed the booking at the time and then emailed to confirm again a few days before the departure date. The email said you do not need to contact anyone, your booking is confirmed. When we arrived at our hotel in Paris they did not have a booking for us and had no rooms available. When we showed them our confirmation from Hotels.com they said they cancelled that at the time of booking due to a problem with the credit card details. The hotel had informed Hotels.com of this at the time.
When I contacted Hotels.com they were completely useless, just telling me all the things I already knew. Namely that they still had it down as a confirmed booking, but the hotel had cancelled it. No offer of any form of compensation for the fact that we lost a day of our short break due to having to find a different hotel. The one thing you want to be sure of when using an agency is that the hotel are expecting you and have a room ready. I would probably have contacted the hotel direct before I went if it hadn't been for the second email from hotels.com telling me I didn't need to. I would never use hotels.com again because you just can't be sure that they are communicating with the hotels they are booking for. Not worth the risk of messing up your break - steer well clear.