I can't comment on placements, but I did my Masters part-time over two years. It was good in that I was able to keep my job while I was doing it and work part-funded me, however I was told up front that I would have no reduction in work - So I ended up doing a full-time job in two and a half days a week and then had to do my Masters work on top. I was also only allowed two days off for study leave, so no reading weeks for me to go over stuff that I was unsure of. Also in terms of other people on the course, there were only a few part-timers so I got to know the full-timers on my course the first year, but the nature of the modules in the second year meant that I got to know practically no one from the second year.
Part-time worked for me in that I wouldn't have been able to afford to take a year off and fund myself, plus I had my job security. But I would always recommend to anyone that if you can do it full-time, then to do it.
It's also one of the reasons why I decided that I wanted to do my PhD full-time and based at the university.