Generally, am a big fan. Even though the trains are a little tired, it does feel like a welcome blast from the past. Being able to leave work in London in the evening, and wake in Aviemore soon after, is a delight.
Sleeping wise, the cabins are too cramped for anyone of average and above height but are just adequate. Travelling as a couple is fine. Travelling solo, with the randomness of sharing with a strangers, takes a bit of getting used to.
On the occasion I travelled EUS-Aviemore in the 'reclining' seats, it was a false economy - constantly playing footsie, noisy, pax boarding in the small hours disrupting what sleep was possible. I've train travelled in lots of places (Egypt, India) and was easily the most uncomfortable journey for the money.
Diner car could do with more staff/choice/room - it's instantly full upon departure at Euston. The poor service chap in there last time I travelled was unable to serve everyone (maybe 20 people?) in the 40 mins we were in there. Totally missing easy money. Barring passengers from eating their own food in there is fine if there is the possibility of getting served, but after waiting 40 mins, we ate our sandwiches and bottle of wine on the padded seats, then retired.
Not sure what the refurb will bring, but price wise they surely cannot put the prices up any more. A late-ish booking last year for two returns from EUS-Aviemore did not leave much change from £500 (£476 I think?). It is a long way I guess, but that was just do-able for me (good salary, disposable income) but I could have bought and insured a beater of a car for a week and driven us up and back for that (did look in to flying too, which wasn't far of the price either).
The difficulty in finding the old 'bargain berths' is also a bug-bear - I'd be inclined to periodically check and scoot up for a few days for a break if it was ever possible to find reasonable prices. Alas, whoever runs the web booking process seems to delight in obfuscating the process to such a degrees as to actually lose the franchise custom.