We have both provided and worked with partners to offer this service to our clients. Overall mixed results. A few key things to keep in mind if you're going to outsource:
1) Make sure they are measuring business (leads) generated from their work. Don't confuse activity with results.
2) Pay attention to the demographics of the users you are seeing engagement from. If your business targets 20-40yr old mothers in California, but your engagement is from 15-25 year old males in New York, that's probably not effective for you.
3) Pay attention to the voice they are using to represent your business. The more people get familiar with social media, the easier it is to tell if the person posting actually knows what they are talking about. Losing credibility in front of a hard-earned audience is never fun and it's always hard to earn trust back.
Overall we've found that it's most effective to train an in-house person to execute the social strategy (which is ideally part of a larger "Digital Marketing" strategy). This way you have someone who is well versed in your business/industry and is already there to help share those impromptu moments an outsourced service could never capture because they aren't in your office.
One great aspect of social media is that it allows you to connect with your audience on a more personal level and show some personality. Be careful not to lose this communication tool by hiring the wrong person/company.