We took the train last summer but took the regular service and only saw the Jacobite train at the station. I couldn't justify the price difference for our party of 8, but it's true that the Jacobite train is more "charming" in appearance (however, if it's not running due to safety issues, it won't be charming at all!!). Our modern train was comfortable if not fancy.
The train ride itself is stunning. Unfortunately the weather was horrible, all rainy the days we were in that area and the day we took the train, but that is just part of the risk in going to Scotland (this was in August).
I definitely support taking the train over driving. You'll be driving a lot already and honestly it gets tiring and is nice to let the scenery come to you without any effort on your part.
I feel like it is better to reserve in advance, but maybe that's just the marketing team doing its job and making me feel like the supply of tickets is limited.