I'd say the model is most comparable to Dropbox in that it passively syncs your work in the background. The check-in, check-out approach works for developers but we found it to be too confusing for our target audience of designers. It's a bit of cognitive overhead to remember to constantly check-in, especially when using Photoshop, which crashes quite frequently.

Every save forms a new version online, but you can go back and retroactively mark certain versions as major milestones. Milestones are roughly equivalent to commits in Git.

Since we're primarily focused on graphic design formats, we don't attempt to do any merging of files since it's unclear that the end product would make much sense. We don't lock files either. If two users simultaneously edit the same file, one of them "loses" and shows up as a Conflicted copy that you can visually resolve in the Comparison view online.