Hey,
I am currently enrolled in this program (international relations research) and I'm very happy that I got the chance to be here. The program has many positive aspects and some negative ones. Whether it's a good program for you or not, is something you have to decide yourself.
LSE enjoys an excellent international reputation in the field of IR. It was ranked #6 best Ph.D. program in IR in the TRIP survey 2009 and #5 in the TRIP survey 2011. It's master's program was ranked #7 in 2009 and #6 in 2011. The fact that it is the university with the best reputation in IR outside the US was the main reason for me to enroll in the program.
Here you can find these survey results (search for "London School of Economics"):
http://irtheoryandpractice.wm.edu/projects/trip/Final_Trip_Report_2009.pdf
http://irtheoryandpractice.wm.edu/projects/trip/TRIPAroundTheWorld2011.pdf
The quality of the lectures and seminars is very high. In my opinion, most of them provide an excellent overview of their respective topics. I believe that you can consider yourself a semi-expert in any given field after attending one of LSE's one-year master programs. LSE has also recently implemented a policy that limits the size of seminars. In most of my seminars we are 10-15 people and have really good and intense discussions.
The student body at LSE is extremely diverse with respect to national and academic backgrounds (the only thing people here have in common is that all of them are good students), and that is certainly beneficial if you plan to study IR. Moreover, London is a fantastic city if you have broad interests, because it has a large number of think tanks, universities, research institutes, NGOs and government agencies.
A slightly negative aspect is that the research track seminars (research design, qualitative and quantitative analysis) are not IR-specfic. The program's coordinators have therefore decided to establish an additional seminar that gives an overview of how these methods are applied in the discipline. I think that was a pretty good idea (but no one tells you about this additional seminar in advance).
The program is quite expensive. And London isn't cheap, too. So you should carefully consider whether you can afford this investment or not. I would not recommend you to enroll in this program, if you're not absolutely confident that you (can and will) continue with a Ph.D. afterwards.
For me the main reason to enroll in this program was that it would increase my chance's to get into a Ph.D. program. I was already accepted by a top-10 program in the US, so it was definitely a good idea to come here. But I think it this decision has to be made individually and you should carefully consider whether it is worth the money to come here.