I've been using Harmony for some time now, but am by no means an expert and have never actually finished a work. There have always been a few things that I couldn't figure out (for example NEB's question about stripping parts for printing), but I've tried to ignore it and put off figuring it out until I, you know, finish something. The electronic documentation that comes with it is pretty labyrinthine and difficult to use: a lot of it seems to deal with basic music theory and defines what a slur is rather than how to actually write a slur! I know what a slur is! That's why I would like to put one in!
Aside from those little things (which, to be fair, are reasonably easy to figure out if you are willing to spend a little time searching through the options) I think it's a great programme for an absolute bargain price. The fact that it's updated very regularly, and for free, is also wonderful. I'm sure it doesn't stand up to Finale or Sibelius but at this stage I'm not in a position to justify the cost of those programmes.