We were in Sintra in May, 2002, and yes I think it was worth it. We were staying in Lisbon for a few days at the end of our Orient Crown Odyssey cruise and took the train to Sintra for the day, visited the Pena Palace, and enjoyed the outdoor cafes.
We will be on Oceania's Regatta on Sept 13 and will again dock in Lisbon. This time we have hired a private guide along with another couple and plan to visit both Sintra and Queluz. This is the other couples' first visit to Portugal and they wanted to visit Sintra and we don't mind returning. We didn't visit Sintra Palace last time so that is probably what we will do while the other couple goes to Pena.
Also, since we will be driving this time instead of taking the train, I am hoping to pass through Caiscais and Estoril. The coast route seems too gorgious to miss.
Enjoy your cruise.