Thanks all for taking the trouble to reply.
As I said, I won't be getting into huge debt as my savings will cover almost the full amount I need. I also don't expect it to give me a sudden huge boost in earnings. Currently I work in a private sector organisation making a healthy $85,000, but longer term I see myself in the NGO / advocacy sector or possible returning to government - I don't expect to get a great deal more than I'm already on in those sectors.
I'm more concerned about the value of HKS outside the US as I have heard others make the same point that Pete does. That said, I have never lived or studied in the US and I was very aware of HKS, which says something (if I was recruiting and someone had it on their CV, for instance, I would be interested). Plus significant numbers of successful HKS alumni have come from overseas, and the Harvard brand is very international. But if any Brits are reading this and have a perspective I'd really appreciate it.
The point is not really to earn masses more money, but to expand and develop my skills, knowledge and networks, and to get my foot in the door of interesting positions that might otherwise not give me a shot. And to have a year living, studying and socialising with a diverse range of inspirational people. I think it will give me those things, but I'm still wavering!