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I've done some co-writing. In the first case, when I have a song that's mostly done and could use some tweaking - I get assistance from another songwriter or two and they help with input that changes the song. In the second case, starting a song from scratch with other songwriters.
I've had great experiences from both. There's a difference between giving feedback on a song and co-writing. Sometimes it's a fine line. However, if the person giving feedback isn't actually writing parts of the song, it's not co-writing. If they're adding lyrics and melodies, it's a co-write. If it's arrangement of musical parts, it's not co-writing - because the song is still the song without that particular arrangement. The song is the melody, lyrics and chord progression.
I find that co-writing from scratch with other songwriters is a great experience, where the co-writers bring into the situation their specialties - the best of what they do - to create something greater than what any one of them could do alone - in the experiences I had at least:)
Co-writing< I find, takes you places you never would go in your wildest imagination- and in an easy and fun manner- for example- someone will say something that will open a whole new vision for you and you can go running off in that direction on a whole new wave of creativity!
Highly recommended ---------------------- : )