I was watching MuseScore for a long time, as Sibelius has become abandonware and Finale hasn't done much progress in the last decades (you are paying the yearly update fee a.k.a. "Finale-tax" mainly for bug fixes…). While MuseScore was at version 1, I couldn't imagine to dive into it, since it crashed far too often – and stability is crucial if you want to do more than just playing around.
But now, since version 2 is out for a while, I can happily report that it is rock stable. I hadn't had any crashes or even bugs for the last months using it. And I have been using it daily, since it (nearly) completely replaced Finale for me. Of course, there are still some music writing features missing that Finale offers, but honestly those are easy to work around using other apps, as anyway for more extended (professional) work one would export and edit the sheet music in an DTP app (and you can just edit the output in a vector graphics app before…). OK, regarding tools for composing and audio, Finale has still the edge over MuseScore – but there are also features that are unique in MuseScore. Support for early music is better than in Finale or Sibelius which were mainly designed with more conventional notation in mind. For example, figured bass is much better thought through in MuseScore, more flexibility, easier to use. Same with different kinds of tablature, for example tablature for lute (which is my field…). While you can do all sorts of things in Finale, it is usually only as an unsupported work around – this makes Finale so difficult to use, and so prone for problems (every update could break your workaround; probably therefore Finale hasn't really been updated for the last 10 years or so, it is stuck). MuseScore is very promising and the UI is built in a way that I think new and more advanced features will fit well into it, especially in regard of audio and composing.
So in the end, MuseScore is just a much "younger" piece of software than Finale is – and since version 2, it has outgrown teething troubles and it shows it's strength, it's fast and easy to use, and yes, feature-rich. Meanwhile, the "big" two, Finale and Sibelius have just gotten very, very old (actually, Sibelius is even dead…), their companies have just specialised in taking our money. So, at present state it is just ridiculous to pay 600$ for either one those. I use them only for legacy files anymore, and every time I have to start up one of those I return happily to MuseScore…
Verdict: I wholeheartedly recommend MuseScore to anyone. Great, great app. Also, if you like it, please don't forget to support the team – my former yearly Finale tax goes to MuseScore now…