I think a day is enough - was for me - except Sans Soucci Park and Palace not much else except the Russian village near the palace. Relocating is such a pain and takes half a day. There is a lot to do in the area but being such a short commute from Berlin I'd do two day trips rather than relocate. Potsdam the city is unremarkable IMO.

One day is NOT enough for Potsdam if you want to see a bit of the city in addition to Sanssouci Palace and Gardens (and you could even spend more than one day in Sanssouci). There are lots more palaces and parks/gardens in Potsdam. But I agree that you can easily do day trips from Berlin, so an overnight would probably not make much sense.

Ah yes, PalenQ my favorite source of information if we disregard spelling or facts. Just ignore his comments about the town. Potsdam is one of the greatest sights in Germany which has the fortune and misfortune to be so close to Berlin. Generally every tourist includes a day trip in his itinerary which is good and usually it stays just one day which is a pity. The point of Potsdam's palaces and parks is their ensemble. It's not just about Sanssouci palace it's about all the structures. And that technically isn't limited to Potsdam/Berlin (though the UNESCO World Heritage site is) but also to other places nearby like Oranienburg and Rheinsberg which give an impression of hundreds of years of a monarchy's representation.
But Potsdam is so incredibly close to Berlin (25 minutes by train) it's hardly worth relocating.