I've done Nanowrimo three times now and found it very worth while. (The only reason I haven't done it since 2009 is simply having no time at all, at the "spending most evenings logged in to work on Remote Desktop dealing with work backlogs" end of the scale).
The whole thing may seem very structured, but in reality the rules are quite flexible - and even then, if you aren't willing to stick to even the very basic ones such as "write a novel", all that means is that you sign up as a "Nanowrimo rebel" and do whatever the hell you want to. (I actually had an idea of something to do this year if I did have time... it would have been a non-fiction manuscript that answers a few of the odd questions I get at work. Maybe next year...)
There's plenty of reasons to have a go, particularly if you fit any of these groups:
* people who have always wanted to have a go at writing, but have never got around to it;
* people who try to write, but always get distracted by other things;
* people who try to write, but get bogged down in spelling and grammar and consistency and all the sort of stuff that a good proof-reading and editing would fix (That is fairly much my main problem).
It's also free to take part, so if it turns out you don't enjoy it, all you've wasted is some time.