We have used the express check-in option for all of our trips, and have usually gotten the exact room we requested. However, on our most recent trip to Pearl, it was like they hadn't even looked at our request. They noted that we had "requested something", but they were not able to accommodate our request. I guess it never hurts to follow up in advance to ensure they looked at your request.
Note: When requesting rooms on what we call the first floor in the US, make sure you use the term "ground floor" or something similar. The first floor in Mexico is what we would call the 2nd floor here. We got caught in that and ended up on the second floor. Luckily, they were able to move us to our exact preferred room after two days, and all was right again after that