We've opted for wedbox. Guests need to download the app or visit the website to upload photos but it's pretty straightforward. They just need the code for your wedding which you get when you sign up. You can invite people to use it through facebook, email, text via the app. We also included the details with our invites and are putting cards on the table to remind people of the details. They do offers via their facebook page sometimes - we got the full package for free form there.
I believe wedpics T & C's mean they can use your photos for whatever they want which is why we chose wedbox instead.
Getting married in September so don't know how popular it will be I'm keeping my fingers crossed we get lots of photos, I've done a photo I-spy to try and encourage photo taking.