First class, as said, is on most Turbostars on the Edinburgh-Glasgow, Aberdeen and Inverness services. On local services, First Class is declassified at discretion. It's also available on any 158s on the above services and on Aberdeen-Inverness runs.
In First Class, you get slightly better seats. On the 158s, there's next to no difference, but the 170 seats do at least look a bit better. I think the 170s have 2+1 seating in the First Class sections.
The Ex-Hull Trains fleet does tend to operate on the Inverness services more often than not, although there's still a few diagrammed for the occasional local run and I've seen them several times between Edinburgh and Glasgow. The buffet in Coach B is locked Out Of Use (although last time I was on board, the catering lady broke in because her trolley broke!). AIUI, Scotrail plan to utilise the buffets more in the future to provide some hot food on selected Inverness services. However, if the units continue to float around as much as they do, I can see this falling flat on its face.