I have to admit I was a skeptic at first. I've used the well known group collaboration / group chat programs out there and just didn't find huge value in most of them other than for group chat. However, with an inbox always alerting me that it has hit the storage limit, I had to find another way. As a team, we are using Spark for fast edits and design collaboration, to track changes and get feedback. It's worked for us!

And looking forward to message markers. :)