While I'm a big fan of La Casa, I was there last month and a majority of the gars they had in that humidor were crap. I dont know what kind of abuse those gars were exposed to during their travels to the store, but some of them didnt belong on the shelves. If you really want to see a pathetic box of cigars, check out the Vegas Robaina on the back of the second shelf beside the humidor door. I almost burst out laughing when I saw those. I bought a Monte 2 there and it was virtually unsmokable. ...and those can be some tasty cigars. I did get a pretty decent Hoyo Edicion Limitada 2002 (I think) there.
At the risk of mentioning a competitor's name, dont be afraid to check out Senor Habanos at the La Isla shopping mall. As best I could tell, the cigars looked authentic to me. They sure as heck smoked better than the ones I bought at La Casa.
There is another cigar shop in the Kukulcán Plaza. I cant remember the name, but I can tell you how to get there. There are actually 4 cigar shops in that mall, but I didnt get good vibes from the other 3. One of the other cigar shops offered to give me fake cigar bands in case I wanted to take any back to the states. I thought that was a little humorous.
My suggestion would be to look at some of Slim's links and do a little reading on counterfeit v/s authentic cigars. Then, when you get to Cancun, shop around and use your best judgement when purchasing. Just make sure you dont buy any from drugstores, flea markets, street vendors, etc.
Dont get me wrong, I buy at least one cigar from La Casa every time I'm there. However, I would not advise you to blow your whole cigar budget there without at least stopping by the other two stores I mentioned above. When you are gonna drop approx $10-$20 per cigar, you might as well explore all your options.
Just my 2¢. Dont be afraid to shoot me an e-mail if you have any questions or want any more details.