I moved from primary to prison teaching (functional skills) six months ago. Before I did so I started this thread to get some ideas on it:
www.mumsnet.com/Talk/the_staffroom/3445838-What-is-prison-teaching-like
It's been a positive change for me but it's not without its issues.
Pros include a great work/life balance. I work 40 hours a week at most and very little of this is done at home - maybe printing off the odd resource, finding a news article to photocopy but nothing at all comparable to working in a school setting.
I also now have an enhanced perspective on life talking to a vast range people who have been through all sorts of things and it's really made me appreciate what I have. I feel I'm making a difference to people's lives.
Small class sizes - I had four men in both my classes today - the most I'll get is eight.
Learners are somewhat lairy but generally pleasant and respectful and are dealt with effectively if they step out of line. I feel safe.
Cons include faintly depressing working conditions. Facilities like toilets and staffroom not great.
It takes ages to get anywhere - if I forget to photocopy something five minutes before a lesson starts I have to unlock and lock my way through about 15 doors and gates - it's a pain.
I work in a remand prison so the cohort changes constantly and learners are all at different stages in the class which can sometimes be quite hard to keep track of. If you can get yourself in a Cat C or D prison the cohort should be more stable.
Spice - it's not unusual to have to send people back because they're off their face at 8.30am. 
Oh yeah - three hour lessons.
Overall though I am really enjoying it at the moment. I may have to look at something else in a couple of years as I'm near the top of the teaching pay grade already but it's good for now.
Hope this helps.