I have done many Panama Canal cruises all with Princess. My friends tease me every time I book another transit. No matter how many times I go, I am still amazed with the locks.
The repositioning cruises are my favorites, as they are longer & often a deal. I too would agree with Tom that San Blas Islands are a must see.
Of course now they are allowing tours in Panama that are quite interesting too.
Good Luck with your decision & Enjoy