For the record, it is definitely possible to only sync the files you want to on your phone - it's what I do. And as an anecdotal data point (FWIW), I've been using it since the Dropbox debacle, with a pretty large data set on weak machines and it's been solid.

Having said that, I completely agree with the open source thing, and am waiting for a good enough OSS one to jump ship.

But for anyone wanting to leave Dropbox, I can say at least my experience was a pleasant one (it's really nice to not have to worry about usage limits, or war criminals).