Ideally, I'd say you want to be in the dome (if it is in your budget) if the dome will be last car on the train.
According to the excursion announcement, all passengers - starting at $329, r/t, including box lunches - will have access to Amtrak's sole remaining dome car, Ocean View. $200 upgrade to a private dome - likely last car on the train - would include premium food and beverage service.
The
Pacific Railroad Society is not only legit, it's OLD. Began in 1936, and has been doing excursions just about as long.
Excursion value is a fairly subjective thing, and everybody uses their own criteria in deciding whether a given trip is worth shelling out, taking time off, what have you. The Tehachapi pass is exceedingly rare passenger mileage. Before the Coast Starlight detour in '08 there hadn't been a scheduled passenger train over the line in about 30 years (several since). The Cajon pass is not rare passenger mileage, strictly speaking (two Southwest Chiefs a day), but riding over it in broad daylight is pretty unusual - excursion territory.
In my view Tehachapi is by far the more scenic part of this upcoming excursion, but Cajon is no slouch, and the desert running on the Palmdale Cutoff is rare passenger mileage too (I like desert scenery from the train, but it's not so hot for railfanning). In my view the scenery is interesting for the excursion's entire run. You couldn't tell that by many of the railfans aboard the CS detour train, though (it was crawling with foamers), most of whom seemed to be making the trip just so they could say they'd done it, and scenery be damned - playing cards, kibitzing, huddled over scanners - until we neared the Tehachapi loop.
For some idea of Tehachapi scenery:
Coast Starlight Tehachapi Detour, 6-22-08. Notes: A shot of the Tehachapi depot shows it after destruction by fire (kids playing with fireworks). It has now been rebuilt and is open. The detour route omitted Cajon pass entirely, departing L.A. through the San Fernando Valley to Santa Clarita to Palmdale/Lancaster (same as Metrolink).
Coast Starlight train 11 waits at Caliente to meet CS train 14, from which shot was taken on approach to Caliente, 6-22-08. Same location, different perspective, as pic posted above.