Winzip lets you connect with Dropbox, but they are not comparable systems really. Dropbox is a great way to spread important files across different devices, as well as keeping a copy in the cloud. Zipping larger files before putting them in dropbox might be a good idea - on the other hand, personally, I don't feel that using dropbox for large files is a good use of the system, though it works great for word documents etc. When you startup the computer (or whenever dropbox is set to startup), it indexes all the files, which can take a while. This can be a little frustrating and slow startup down considerably.
As to actual file sharing to other people, dropbox
is no good (unless you're happy to all share the same folder), though it serves as a great system for oneself. Sharing files on google docs is probably one of the better methods I've used, where it's possible to contribute to a shared document and annotate each others work.