I did.

They do prefer you to do it online though.
I would suggest going when they aren't busy. CineWorld aren't known for having lots of staff available.
If you do manage to get them to do at the cinema it make sure you take all the ID, bank details if you are doing an D/D etc. You'll see what you need on their website where you fill the form in. I'm not sure how many passport photos you need. Definately two, maybe three. When they do it you'll be given an temporary pass which you can use until your proper one comes.
One thing it's worth knowing about the Unlimited Pass. It says that you can only get tickets on the day of the screening you want to see. This isn't always the case. I often get all my tickets for the week in one go. It's easier to do it that way. I've been going 3 times a week and get all my tickets on the Friday ready for the following week.
I've only been told no once by a jobsworth who was new so was doing things correctly. The rest of the time the staff don't give a toss and just give you the tickets. You may be pushing it if you want tickets for the busy period though.
It's just one of those ask and see situations. Be nice, ask how they are doing, get talking to them and they tend to be more helpful.

Like any service or retail job it's can be a crap job at times where you don't get treated like a person by so many customers that when people are nice and friendly it means a lot.