Pros: MuseScore enables you to write music scores of any length for virtually any instrument or combination of instruments, and you can play them back right through the same software. You can also export them to audio files, other music software formats, and even PDF files. In their native format, the scores can also be uploaded to musescore.com and shared with others, who can not only read your compositions, but also play them back right on the site and leave comments. The software can even alert you if the notes you're entering are beyond the range of the instrument, so it's an educational program, as well. MuseScore's developers maintain a good forum if you need help.

Cons: MuseScore's learning curve is a bit steep, and of course you need some basic knowledge of music notation and music theory, but if you already have that you'll eventually get accustomed to the software and find it indispensable.

Other Thoughts: MuseScore is currently the most important application on my computer. I can personally vouch for its reliability, because I've successfully completed over 200 scores with it over the past year and a half.