Sibelius has been around in various versions for a couple of decades, and there are no other programs that even come close to what it can do. Sibelius First is an affordable little brother program to Sibelius itself, with virtually all the functionality, but at a fraction of the price. It's the musical equivalent of a word processor, making the process of composition or transcription as painless as it could be, and outputting professional-quality scores, either as printed parts or for sale over the internet. It can also read in and output to MIDI files, and to CD audio.
Sibelius First uses the notation of musicians, not of computers. You write in ordinary musical notation, and you have full control over the layout and appearance if you wish. Sibelius will keep an eye out for what you're doing, and will automatically avoid congested or unreadable scores. The nice thing is that Sibelius First understands the score - it keeps lyrics, tempo technique and expression markers in the right place, and it obeys the instructions when playing back (apart from singing the lyrics - it can't do that)! It also supports appogiaturas, tremolos, upbow/downbow, and up and down arpeggio markings, and double sharps and flats.
Input is mainly keyboard-based. You can use the computer keyboard to enter notes or chords, or you can use a MIDI keyboard or guitar, either to play a part live or to input individual notes or chords. There's a plug-in called "renotate performance" that helps to sort out any irregularities in your playing, and make the notation easier to read. It is absolutely vital to read and work through the three tutorials in the PDF manual (it took me about a day, and was well worth the effort). Sibelius First is easy to use, but you need to know how. If you try using the mouse for everything, you won't be getting the full benefit of the program - the keyboard shortcuts are MUCH quicker.
You can print out both a full score and individual parts. If any of the parts are for transposing instruments, that's all handled by Sibelius First. You can edit the transposing part if you wish, and this will automatically update the full score version (actually, the parts and the full score are all stored in one file anyway).
What, then, are the limitations of Sibelius First compared to its big brother? There are a few, but just three that affect me. Firstly, you're only allowed to install it on a single computer (Sibelius allows two). So if you normally use a desktop computer, but want to take the score to band practice in case you need to make any last-minute changes, you can't. It's probably best to use a laptop anyway (but one with a reasonably high-resolution screen). Secondly, you're limited to sixteen staves (Sibelius is unlimited). For most amateur musicians, this won't be a problem, but if you want to load in a MIDI file of, say, "Spem in Alium" to study the parts separately, you're going to be disappointed. Thirdly, the shortest note allowed is demisemiquavers, and the longest note is a breve (whole note), so scorchingly fast violin solos are probably out.
I'm using Sibelius First on a Windows 7 64-bit machine. It's an absolute delight to use. I can't recommend it enough.