I did 2 years as a crown servant and I'm coming up to 2 years as a civil servant after my role swapped home organisations. Currently trying to move on within the CS.
Perks
Good holiday, good pension, family friendly inc mostly flexi friendly- although you get the benefit even more if you're not in a role that means you have to be in the office due to nature of docs handled etc. One of the reasons I'm hoping to stay in is because I want a family. Also perks like discounts etc which all add up. Options for training, too.
Cons
It's not the most ambitious culture so you have to stay motivated and push. I'm trying to move to a dept with more opportunities and linear progression possibilities; there's nothing where I am. It's easy to get stuck if you don't work at it (although I stayed put longer on purpose as I wanted the security in the pandemic). The pay isn't the best but it's still ok. The worst working bits, as highlighted by others are dire tech and a lack of support/people- my HR is self service and it's awful. Trying to get someone to fix IT can be maddening- you're a wee cog in a huge engine, good luck getting through to a human. I'm still waiting on 2021s p60 (I've asked 3 times). I've cried with frustration and fury at wasted time before. If you don't have a supportive manager, it can be lonely, trying to get on, too.
Most of all, it's slow; especially recruitment.
My recruitment as a crown servant was easy peasy but I'm currently waiting on a contract in the post so I can move roles after a 9 month recruitment process. If your job requires vetting, it can take up to a year. Be prepared for v minimal contact whatever- if recruitment processes were the dating game, the CS is a fuck boy. See also, some departments having weird application processes (looking at you, national archives who want a bloody ucas statement for every job).