The quick answer -- they are great for some things.
--Backercamp's basic service ($19) is EXCELLENT, but should be done well before launch. They looked at my whole campaign, gave MANY helpful ideas ranging from identifying/addressing my audience (parents with young children) to changing the copy in different sections of the page to redesigning the look (adding jpegs as title cards for the different sections instead of using KS's text tool and creating a visual chart to show the various rewards). They answered all questions quickly and completely (even on weekends).
--However, I got no backers from their retweets. Zero. Maybe my particular campaign--3D animations for children's songs-- didn't match what their followers are interested in.
--I paid $129 for them to send my press release to 50-60 bloggers/journalists/etc. This was a TOTAL WASTE. They forwarded my press kit to 56 blogs. No one responded--not even a rejection. I asked Backercamp for the list of blogs they wrote to, and upon seeing it I understood why. Only 6 of the blogs were appropriate for my project. The other 50 were either too popular (NY Times, HuffPost), the wrong audience (high school teaching materials, things to do in Chicago), or totally inappropriate (a civil rights information website, a book review site, a business journal, an environmentalist blog).
--I thought that for $129 they would vet the blogs before contacting them. Instead it looks like they blindly emailed the first 45 results of a google search, then threw in some famous magazines on top of that. A big let down.
--To be fair, their initial work far surpassed the $19 price tag. I feel proud to have shared that campaign with my friends and family -- it was definitely better than my first draft. Their failure with the PR campaign may have been due to my project's topic. But if they were having trouble finding appropriate blogs, they should have said that and refunded the money. When I told them I was confused by their choices, they said everything was related to the keyword "parenting", or each site had a parenting section. Okay, but while Publisher's Weekly does have a children's section, they focus on children's books, not Kickstarter campaigns for children's music videos. I was not impressed by their answer, and they did nothing to try to make it up to me.