Put it another way, I don't understand why you'd use Dropbox at all, after all you could easily version all your files in git, using push/pull/branch/commit/rebase to get your data to your different machines. Git handles this problem very well, after all.
Obviously with regular files, that is not an issue, so Dropbox works well. (unless you're managing them with git, I guess)
If you're willing to accept the "lightness" of git branching and commits, it's possible to "save and sync" your work very quickly. Assuming you're on a dev branch:
git add -A
git commit -m "temp commit 2013-7-3"
git push origin
You can even script this, see for example:
http://stackoverflow.com/questions/8482843/git-commit-bash-s...
Then when you're ready to work on a different machine, you fetch from origin.
If you want to keep your commits meaningful on your dev branch, you can create a temp branch first, then rebase later to get rid of the ugly temp commit.
git checkout -b temp123
Dropbox could help there, but it does not.
I guess you could say Dropbox gives me finer sync granularity than git does.
Dropbox seems like a perfect solution: have a single place where you keep code, develop on two machines, just pick up your laptop and leave whenever you want, without worrying about sync.