I regularly travel between Euston and Inverness, and it's a great service ... and has recently been fairly reliable to boot!
There's usually quite a good atmosphere on board, and the crews are generally very friendly. (There was a time when we were raided by the BT police at Preston, and a couple of guys got frogmarched away, but let's not go there!)
I understand that there is an obligation on Scotrail to provide the current service as part of the franchise obligation, and they do promote it sometimes, but I often feel it's underutilised. The carriage sets are showing their age, and must have clocked up an enormous milage - they could really do with refurbishment or replacement, and do suffer from a lack of storage space, especially for bikes.