We only chose S7 Airlines because they are part of OneWorld and we could accumulate tier points/avios - it was more than double the price of Air Astana and I had a way better experience travelling with them on my outbound flight so I'm quite gutted I chose S7.
The only benefits we got as being part of the alliance/silver members was that we got to choose our seats upfront, but because S7's system is so appalling I had to email the Priority team to amend it manually, this however then removed my British Airways Executive Club number and turned into a nightmare.
The check in process is relatively simple, hand luggage racks are always rammed so get there early, however everybody tries to board at once due to a lack of organisation and this urgently needs resolving to make it a smoother process.
Although I had a generally good experience with cabin crew, there was 1 in-particular who made me feel very unwelcome, she looked at you like a piece of dirt, refused to speak to you in English and snarled at you the whole flight when she walked past.
I wouldn't fly them again due to being disorganised. I also didn't feel very comfortable transiting through Moscow.