Shutterstock:
Likes: Great sales, quick pay, excellent forum, fast review time, FTP, easy keywording, many different formats accepted for video clips.
Dislikes: Rejection reasons of late have been peculiar, site navigation could be better.

iStock:
Likes: Good sales, detailed stats, easy pay-out system, accepts wide range of imagery from stills to video clips.
Dislikes: Disambiguation and keywording is a pain, the whole "exclusive" thing, no FTP for images, or nonexclusive videographers, restricted speech on forum, rejection reasons sometimes weird, hard to get a 3D renderings accepted.

Dreamstime:
Likes: Good sales, FTP, easy keywording and categorization, Ellen's idea-loaded blog, nice site navigation, good forum, easy pay process.
Dislikes: Never had a response back from support on any question or issue sent to them, editorial pics require release forms, speculative definition of what is defined as an editorial photo.

Fotolia:
Likes: Decent sales, FTP, excellent acceptance rate of submissions.
Dislikes: Wonky categorization process, for me older images sell far better than newer ones.

Stockxpert:
Likes: Steady sales, FTP, fast review time, easy keywording and categorization.
Dislikes: Unhelpful rejection comments.

LuckyOliver:
Likes: Run by nice, friendly folks, FTP, excellent site interface, easy keywording, nice blog, happy-go-lucky atmosphere.
Dislikes: Almost nonexistent sales.