I' ve been on two, and had decent experiences with both. I would say that we paid slightly more to narrow down the options both times. One was in the caribbean, where we knew it would be one of 5 luxury hotels, and we ended up in the second nicest.
The other was in portugal, and we paid a little extra to guarantee resort and 3star s/c minimum and aircon. Like Mrs O, ended up with a lovely big new apartment. Couldnt complain.
We felt it worth paying that bit more for a little more certainty, especially given some other people we know have had some iffy experiences with tiny apartments, no aircon in crappy noisy resorts.