I fully agree with Alongor that the STP is the best way to go if you are exploring much of Switzerland and constantly moving daily. However, if you are based at a certain location for a long period, it might not be the best. The half fare pass would probably be the better option.
I had the 15 days STP and traveled from Geneva to Lausanne, Montreux, Golden Pass line, Thun, Spiez, Adelboden, Kandersteg, Brig, Lauterbrunnen, Murren, Gimmelwald, Grindelwald, Schynige Platte, Interlaken, Brienz, Lucerne, Zurich, Bellinzona, Locarno and Lugano.
I cruised on Lake Geneva (Montreux - Chillon return & Cully to Vevey), Lake Thun (Thun - Oberhofen - Spiez), Lake Brienz (Brienz - Geissbach See - Iseltwald) and Lake Lucerne (To Vitznau and back from Weggis). Could have done a cruise on Lake Zurich too but since December 16, they have introduced a surcharge of CHF5 per passenger. I know its only CHF 5, and worth paying for, but i was getting off on the next stop, so didn't take the cruise.
Entrance to Chillon, Oberhofen, Spiez and Bellinzona castles were all included in the STP. Covered Mt Pilatus and Mt Rigi as well. FIFA museum in Zurich was included.
The best advantage is that you don't need to worry about buying any ticket, just check the train times and the platform and hop on and show the STP to the conductor. As mentioned by Alongor, you have to keep the ticket safe/dry/wrinkle free as sometimes, they would scan to check for authenticity.