Me and my partner have done the Caledonian Sleeper to and from Aberdeen once and Inverness three times, in both first and standard class, and we find the service fantastic.
First class - very enjoyable, the extra space in the cabin is great, the first class breakfast is also a lot better than the standard class coffee and shortbread, a bonus of first class is being in the sleeper coach closest to the lounge car, so if you plan to take advantage of the lounge car, it saves staggering back down 2, 3, 4 or more coaches back to your berth. The little gift bag of tooth brush, tooth paste, shaver etc is a nice touch.
Standard class - although there is the second bunk in use its still a very manageable space, but each time I have shared a cabin it has been with my partner, I would never share with someone I don't know. As mentioned before the breakfast has been downgraded from a continental style breakfast to just a hot drink ans shortbread, On a personal note I don't see that as a problem because the continental breakfast was to dry for me and I only ever fancied a coffee and shortbread after being in a cabin all night.
Overall I would recommend this service to everyone, I would do first class at least once, but the service on a whole is very expensive. Like I say, id go first class just to try it but after that I would check out bargain berths, basically the same as standard class but you can pay as little as £19.
The Inverness portion seem to be busy more often than not, but the Aberdeen southbound portion was very quiet last time I used it. Like us, if you plan on making full use of the lounge then avoiding the Inverness portion seems to be the best way, or make sure you are there as soon as the train is ready for boarding, get your stuff in your berth and get in the lounge ASAP.
Overall, a fantastic service, the lounge car service is always exceptional and sleeping have never been a major problem. For me personally, when in standard class I would be on the top bunk, and find it a little harder to sleep there, plus the fact I am scared of missing anything! but my partner sleeps like a baby on the bottom bunk. When in first class we both sleep fine on the lower bunks.
The major plus with this service is the fact you are buying yourself an extra day in the highlands on the day you arrive and depart, especially if you manage to get a bargain berth, and I have never stayed in a guest house that has not been willing to take our luggage in early and look after it for us until we depart in the evening.
The only downside is the high price, 2 adults with a disabled adult railcard in first class is costing us £331.20 EACH WAY, where as the Night Riviera is around £60 (off the top of my head) per person each way, in a solo cabin with TV and a good looking breakfast and lounge car facilities. For a cheap trip to the highlands, take the plane, if you like your railways, enjoys a nice long ride on some of the most scenic lines in the country, then take the Caledonian sleeper!