There is a lot of self-study as it's at university level. Most students do work full-time and have 'a life' outside uni (e.g. a family, hobbies, other commitments) which does make it easier as people are very understanding of each other's situation and how it can affect your study. I've found the teaching to be set up to support this as well and the evening classes work out just fine IMO. You become very good at organising your life by doing it this way!
I've found all the students on my course to be very motivated and I think it's because most us (me included) are mature students who have been out in the workplace for a while and we know that we are doing the course for a particular reason. It can be hard going at times, but you just have to remind yourself of why you decided to do it in the first place and that is to maybe further your career, learn more about a subject you are really interested in or to just challenge yourself. I read somewhere that the average student age at Birkbeck is 35, but it ranges from school leavers to people in their 70s as well and there are quite a few international students on my course too. I see that as a great positive though as it means the discussions we have in class gives you many different perspectives and you can learn a lot from hearing other people talk about their own experience.