How about that for a mouthful? I have actually used all three. I use Dropbox the least amount of time however most admin. & teachers in my district use Dropbox more regularly than I do. All are accessible on the iPhone.
Evernote: My scanner is set up to scan all paper to Evernote automatically. Any time I get handouts, meeting agendas, teacher notes, etc. these are scanned into Evernote. My scanner is the small scanner and I will probably invest in the larger one in the fall. The hardest part is remembering to go back into the "note" and give it a title and tags so I can find it later. The search feature of Evernote is amazing so if I forget to title or tag I can search for any word in the document and eventually find it. (Tune in for more posts this week regarding Evernote. There are so many uses!)
Google Apps/Regular Google: This is where I live. Now that we can upload ANY file to Google apps/documents this makes Dropbox irrelevant to me. I can upload my files and keep the original formatting I just have to remember that if I downloaded I have to re-uploaded it as the new file. Have not had to do that yet. Now that Google docs has been upgraded and they added Google Drawings I can do just about anything I need right in Google.
Dropbox: When Dropbox first came out I used it for a couple of documents to work on at home. I think my entire "My Documents" folder is sync'd to Dropbox but I have not used any of my documents in there. Some other principals have share some Dropbox folders with me but that is the extent of my use.
This is a very brief comparison but I have to get home because my sitter leaves in an hour:):):) Still trying to post in June though!