After several trips to Switzerland and many train rides in other areas, we finally got to ride the Bernina Express---the route on a regional train, not the "real" one. We were actually headed to Venice but had some extra time, so we flew into Zurich and took the grand directly to Samedan, a small village between St. Moritz and Pontresina. We spent two nights there to have a full day hiking, and fortunately had glorious weather although it already October. The next day we boarded the train for the ride over the pass and down to Tirano.
It was every bit as spectacular as you have heard, far more so than the other so-called "scenic" train rides. While those have sections that are very nice, the Bernina Express was non-stop gorgeous views. And because the train makes switchbacks down the mountainside on the South side, you will often pass the same view, first on one side of the train, then on the other. Fortunately our car was nearly empty so I could bounce back and forth from one side to the other to take photos.
If you do this I have two tips----if coming from Zurich, there are two routes to Samedan, both covered by the same tickets. On the train we were advised by the woman sitting across from us to take the more western route, via Thusis Tiefencastel, not the eastern one. This was spot on, as the train actually follows some of the Glacier Express route, including the famous high stone bridge ( whose name escapes me just now).
Second, sit near the back of the train and you will be treated to a view of the train itself going around sharp turns, including the above-mentioned stone bridge. Great photo ops.