I've been using Guitar Pro pretty regularly for the past 3 years to compose some of my music. It was originally designed to be a guitar tab editor, but it's pianists and violinists would find themselves right at home as well. It's pretty convenient, but I often find myself complaining about some little thing that Visual Studio has that Guitar Pro could really benefit from. This is coming from a programmers perspective, but these would make very good features in an "Advanced" menu.

• Commenting: Commenting is by far my most sought after feature in Guitar Pro, though admittedly commenting a music track would add a lot more complexity. Normally when I want to save a riff for later, I put it at the far end of the track, meaning by the time I'm finished with the song, I'll have 5 or 6 snippets that don't really belong anywhere but "comments".
• Batch Modify and/or Find and Replace: I can't count how many times I've wanted to a do a pattern match or a find and replace on my guitar tracks. As it stands, there's a lot of manual (see: repetitive) that labour involved in laying down a track. It's a matter of adding nice (even vim like) commands that would really speed up that process.
• Export to MP3 Rendering: Guitar Pro features a rendering library for guitar, bass and drums called "Realistic Sound Engine" or RSE as an alternative to MIDI playback. The quality is usually pretty nice (though there a few volume inconsistencies with the MIDI player that drive me insane), however that's no "real" way to export sounds rendered in it. I put "real" in quotes because there does exist a Export to WAV option that would do essentially what I want, however it just tries to record the output of your sound card. This can be problematic since some sound cards don't support recording their output. What I'm looking for is real FL Studio style rendering that has no sound card in between it and the resulting MP3.

Guitar Pro does get a lot of things right though. It does have a few nice keyboard shortcuts, such as C to copy the current note to the end of the measure, Insert to add a rest before the current note with the same value as the note, CTRL-Insert to add a measure before the current measure, P for palm mute and I for 'let ring'. It also supports pasting multiple times with a variety of options (insert, paste over and add to the end of the score). Guitar Pro also uses numbers for the drum parts which are easily memorized. The built-in scales and chords are also nice when I'm having musician's block.

There are a few alternatives out there: Power Tab, which is free, and TablEdit. Hopefully the pressure of competition and the release of Guitar Pro 6 will add some of these features! They can all learn a lesson from IDE's to improve musician productivity.