Spent a few nights here last week. Overall, very pleased with both the property and Montreux.
Was traveling with family, so had reservations for two rooms. Used SNA on one of them; didn't clear. It was a midweek stay in shoulder season, with reasonable rates and all room types open weeks prior, but day of arrival, the place sold out. So my terrible SNA success streak continues, as does my history of mediocre to terrible Marriott upgrades that spans all of 2023. We got two "partial lake view" rooms on the second floor. The lobby is on the 3rd floor, so you take the elevator down. That sounds worse than it is -- the view was quite nice, the room larger than some of the higher floor lake view ones, and the patio was fantastic. No issues with road noise at night. We were directly below the Grand Suisse logo; only four rooms have balconies as large as this. Was quite lovely in the autumn sunset, and I might be tempted to pick this room again over a higher floor with a less obstructed lake view, but almost no space to sit outside.
We had dinner at the hotel restaurant; food was decent, prices reasonable (by Swiss standards), and the view very pleasant. Free elite breakfast was granted for both rooms. A buffet covering the basics, decent for Europeans (meats/cheeses/breads), but uninspired for Americans -- the hot items were bacon, scrambled eggs, two kinds of pale looking sausages, mixed vegetables, white rice and haricots verts, a very odd selection. No variation over the three days we were there. A bottle of cheap sparkling wine was available, and some guests happily partook, mindful, perhaps, that a .1l glass of middling wine easily runs $10 in these parts.
The tourist tax pays for a card that entitles you to free local transportation and discounts on attractions; make sure to pick it up from the front desk. Montreux is certainly worth a visit, and it's hard to fault the Grand Suisse if you can get it for 200 CHF or so; several hotels were selling at 5 times that rate, and for the life of me, I couldn't tell what justifies that difference in price.

Partial lakeview room on 2nd floor (tiny bathroom not pictured)

Balcony

Sunset from partial lakeview room