Sibelius provides music notation software for musicians of all genres and levels., whether you're a professional composer, or just a casual one; a music student or a professional.

Personally, I have found Sibelius to be the most easy to use software, in comparison to the others I have tried. It allows you to transform your compositions into professional looking scores, work from a transposing score, automatically print transposed parts, there's an ideas bank to save motifs/melodies in progress/random ideas you want to use but not yet, or want to work on before you add them to your composition.
Sibelius 5 also ships with far better sounds than on previous versions, meaning you can hear your compositions played back without frustrating midi sounds!

Printing and copying parts is so easy, and once you've got your head round a few keyboard short cuts, composing and arranging is quicker and easier than ever. I really strongly recommend learning the shortcuts for keyboard if you can, it takes a bit of time, but once you've got the hang of it, it's so much quicker than using the mouse!

I've found Sibelius is great for school and college work, in all areas of Music, and will definitely continue using it as I progress onto university! And if cost is an issue, (it's an expensive piece of kit!) they also do an educational version, or the full version at an educational price. Site licenses are available for schools too. And once you've bought a version, upgrades are much cheaper, (upgrades are around about £100ish I think (including educational discount,) meaning you don't have to pay out hundreds and hundreds each time there's a new version out.

Set up is really easy, just install and register and you're pretty much ready to go! And the support website has been very helpful on the rare occasion I've needed to use it, even if only for advice about how to find something or work a feature more productively.

One thing to bear in mind though, is that Sibelius 6 has just been released, so it might be worth waiting for that...I've just started using it, and the benefits and new features are really worth having! (There isn't a dooyoo listing for it yet, but if my suggestion goes through, I'll review that too!)