I took it just a couple weeks ago from Saint Paul to Seattle. You might want to check out some threads on tripadvisor's train forum. The insights there were more or less confirmed by our trip.
The best things about the trip were the people and the food. The people were incredibly friendly; we sat with different folk--couples, singles, families-- for each meal and in the observation car and everyone was unfailingly nice and interesting. Heard many good stories! Also, the food was excellent. Three big meals per day came with our roomette--a menu to select from for each meal with fabulous selections, including superb steak and seafood. We also had a fun wine and cheese party.
The not-so-great things were, first, the tiny, tiny roomette. I'd always spring for a larger room next time. You can't even breathe in the roomette, although we did sleep pretty well. The second not-so-great thing is the scenery. It is, frankly, very boring through the vast majority of MN, ND, and Montana. It only got interesting toward Glacier, although we were 3 hours late by that time and it was already too dark to see. It was interesting, then, the next day as we entered the Cascades.
Lots of people were making the trip to Glacier and got out just as we were bedding down for the night. It appears that the train is nearly always late--the oil boom in Williston ND is hogging the tracks, evidently. So plan accordingly.
All in all, we were glad we did it. But we flew home from Seattle and were also glad we had planned to do that.