Very Powerful for the Money
"First" is a budget-minded version of Sibelius with the more advanced features stripped out. All of the commonly-used features are there. So far, I have only run into one or two features I wish were included, but they weren't critical. On Avid's web site, there is a detailed feature comparison matrix that shows which features were included or left out of First.
I am a Pro Tools user, which has a very basic score editor that can't do a lot of the things I needed (like tablature, repeats, etc). Sibelius First does those. However, I am disappointed in the lack of integration with Pro Tools. Pro Tools has a "Send to Sibelius" function, but with Sibelius First installed, Pro Tools doesn't recognize it. I can't even cut-and-paste. I have to export and import MIDI files.
I was pleasantly surprised with the ability to enter notes from a MIDI controller and to play the score on my audio interface. It just works, without any complicated setup at all.
Sibelius First comes with a piece of software to scan printed sheet music and build a score from it. I tried it once, and it was very inaccurate, so don't buy it for this feature.
Sibelius is a powerful program, and the learning curve is rather steep. Though it does a lot of things for you, learning how to do things differently can be a challenge. I have had to rely on 3rd-party training videos, but I'm getting there.