I know this doesn't directly answer your question, but I first want to say that you DEFINITELY do not need a 750 GMAT score to get into HBS. The majority of students at HBS (or at any school) have a score below that number. If you're thinking that you need a really high score to overcome a weakness in another part of your app, that strategy usually doesn't work (except for in a few situations such as when you have a poor undergrad GPA in quant-type courses). If you have an application with some serious holes in it, adcomms are unlikely to turn a blind eye just because of a good GMAT score, no matter how high it is.
Having said that, the best advice that I can give you is to start early, and to spread your studying out over a long, long time. (It sounds like you're doing this.) Books are enough for a lot of people. I only used books (specifically a Kaplan book and the Official GMAT Guide). For others, a course is good because it helps them focus on a particular area, and it forces them to stick to a strict schedule. This latter point is important for busy people or for those who have a hard time gettingt motivated to study.
I didn't use a course, but I mainly know people who have used Princeton Review, Veritas, and Kaplan, and they have usually been satisfied. In instances when someone wasn't satisfied, it was usually because of a specific instructor. Integrated Learning is another GMAT prep company that we partner with. I don't know any of their customers personally, but they're pretty well regarded.
I did use some other web sites and free software that I found on the Web, but in many cases the questions weren't written very well (i.e., they didn't mimic actual GMAT questions accurately).
Good luck,
Scott