The lock-in with DropBox is huge - I have eleven shared folders that I use to keep other people up to date with files, and I have never had a request to share files with anybody on anything other than Dropbox.
I trust dropbox. Dropbox has a great track record (with me). Dropbox is installed on all my tablets/smartphones/desktops/laptops. And, at $100/year, it's cheap enough that I'm not really inspired to bother price shopping.
This is clearly a case where Dropbox came in and won - and now it's time for them to invest aggressively, and make sure nobody else enters this market.
Now - I am not saying that people aren't using other systems - obviously Microsoft and Google have their products, and both Google Drive and Microsoft Skydrive are excellent products, that are technically comparable to what dropbox is offering. Apple's icloud is just annoying for knowledge workers, so I'm not going to include that in the same class - but, in terms of a generalized folder Sync Product - Dropbox has won.