As discussed in this thread, on Citynightline routes* I tend to use 4-berth couchettes (if travelling alone), or 2-berth sleepers (if travelling with Mrs KQ321). The beds in the sleeper compartments are definitely wider and more comfortable than in couchettes, so I will occasionally take a shared 2-berth when travelling alone, but I am normally content in 4-berth couchettes (6-berth can be a bit crowded). There is generally a significant price differential between a 2-berth and a 1-berth sleeper, so I don't think I've ever paid extra to have a compartment to myself (although once or twice I've got lucky, and booked a 2-berth compartment, in which the other berth wasn't used).

I've also never found the 'sharing with random strangers' bit to be a problem. Everyone makes an effort to fairly share the limited space in the compartment, and people tend to chat a bit (how much depends on whether you have a common language) - but otherwise people leave each other alone to get some rest. Indeed, given that people get on and off at different stops during the trip (but each berth is only used once during the journey), on some occasions you may hardly see the people you're sharing with (if you're asleep when they get on, or vice versa). If I have a choice, personally I prefer an upper berth, as then you have it completely to yourself, while lower berths may need to be shared if other people want to sit during the evening/morning - but even that is rarely a problem. Overall, a 4-berth couchette compartment feels far more civilised than being squeezed in 10-abreast on a long-haul overnight flight in economy...

* I would include the Berlin - Vienna Metropol in this (even though it's technically a Euronight service) as the pricing structure is similar to Citynightline, albeit slightly cheaper.