Sabes44 -- I'm using destinationweddings.com for a May 2015 wedding in Mexico (ALSO at Excellence Riviera Cancun!). My TA is wonderful. She's very attentive. Even after we went through all the buzzy activity of narrowing down resorts, getting quotes, and finally making a selection, she still calls me once every few weeks, just to check-in. I figured I wouldn't hear from her again until payments were due - but nope, she calls and asks how we're doing and how dress shopping is going, etc.
That being said, I did not agree to do the room blocking contract. It put way too much financial obligation on us for guests who may or may not decide to book. I felt like we'd have to chase folks down and force them to book their rooms, which is definitely not the sort of overall "vibe" we want for our wedding! If they want to come, they can book through our lovely TA. If they don't want to come, that's ok, too! If they want to internet search for cheaper pricing and book through Orbitz or whatever, they can do that, too. Most importantly, when we had a zillion questions for our TA regarding the contract, she gave us thorough answers, helped us weigh the pro's and con's, and didn't pressure us one way or the other.
Depending on the resort you've chosen, maybe there are better incentives for blocking the rooms upfront. Our resort did not offer significant discounted pricing - only room upgrades for me and my fiance for every 10 rooms booked. If there was some sort of across the board, significant discount on pricing that was shared with all of our guests for blocking the rooms in advance, then that would have been different.
So far, I'm still happy with our decision(s). My guests like the special attention when they contact our TA for quotes and they definitely appreciate the payment plan. The DW website is also very convenient for sending save-the-dates, tracking bookings, and sending mass-communications. Anyways, fingers crossed & good luck!