Hans: a visit to Queluz very much depends on how much time you have for that one day visit to Sintra - there is lots to see in Sintra, Pena Palace, National Palace and Quinta Regaleria are the 3 big ones.
That said Queluz is well worth the visit, plus the opening time is now at 9:00 and the train passes the stop on its way to Sintra.
They call this the Portuguese Versailles and it is a very much smaller and more manageable version with its little mirrored hall and its gardens laid out in the style of Le Notre with a Portuguese twist of a pleasure boating canal laid out in tile work.
The big plus is that this Palace is furnished unlike the French version, so look out for the matching star burst crystal chandeliers, well used Chines import dinner services, nativity cabinet - try and find the women hair pulling fight at the side, the mix an match Virgin Mary sitting on top of a Hindu throne and the Dom Quixote bedroom that looks round but is really rectangular.
So Yes if you have the time to spend the Palace is well worth the visit. Actually there are many places not mentioned on the usual tourist list that are more than worthwhile visiting.
If you need any other help please just send me a private message, and if you do go let us know on here how you liked Queluz. Sunny regards from Lisbon. your-lisbon-guide dot com