I’ve used them a few times. The best that can be said for them is that they’re OK, though to be fair the staff are always pleasant. The biggest problem is that the trains they run are essentially designed for suburban/outer suburban journies and are really not up to long distance travel.
I’ve always been offered a free (not very good) sandwich over and above the tea/coffee, though maybe they’ve cut costs since.
If you want more comfort, you’d be better off using a combination of East Coast and Virgin, but maybe the times will be less suitable for what you have in mind.
If you go via Glasgow, you’ll have to change stations (10 minutes’ walk).