We've cruised the Mexican Riviera a couple of times on Princess.
The last one was a ten day on Sun Princess.
The seven day cruises have basically the same itineraries, Cabo San Lucas, Mazatlan and Puerto Vallarta.
We love all three, especially Cabo. The Mexican Riviera is nothing like the Caribbean, IMO. The towns are really interesting to walk around with lots to see. It's also pretty inexpensive.
The problem with the seven nighters is that they have a very short season, like October through May. The three and four nighters sail all year round. If you want to stay away from the "booze cruise" crowd, you can sail RCI's Monarch, which appears to have a better reputation than the Carnival Ecstacy.