Just did this myself last week. The seats don't recline much and you're at the mercy of the gods as to who you're sat near - a weirdo who goes to sleep before 9pm, requiring you to sit stationary and silent all night; a fidgety, weak bladdered weirdo who will keep you awake all night; a [censored] with crap music blaring through their headphones all night, despite having fallen asleep before the train reached Lancaster... The atmosphere in the seated coach can be a little weird, and you're very aware of the need to keep quiet. I felt guilty going for a wee. I wouldn't want to do it often but I got from Crewe to Aviemore overnight for £55, which was worth the discomfort of the experience.
The cabin berths are obviously much more comfortable but also much more expensive.
The sleeper train is still a great experience though, you go to sleep with anonymous, fluorescently-lit towns and factories flashing past, and wake up looking at the Cairngorms under dawn skies. Magic.