Well, there are plenty of comments already in the forum on Dropbox - a little searching may provide you more details.
Also checkout the short video HERE at their website; the program needs to be placed on multiple devices to 'share' your files 'in the cloud' - I've just started to use it, myself - good luck!
Depending on what your use of the iPad is, Dropbox is fantastic for accessing files on multiple devices. I save files to Dropbox on my work computer and I can open them and alter them on my iPad, my home computer, or even my phone. Great app.
I agree. To save space on my iPad I am constantly moving documents in and out of my dropbox when I travel for work. It works fine for me for now since I haven't yet paid for the service.