I've been a customer of Channel Advisor for over 6 years. I decided to use them initially for our eBay business and currently use their Paid Search, Shopping Feeds & Premium store solutions.
It's like having a fulltime software engineer on staff but 24/7. eCommerce is continuously changing and keeping up with all those changing is time consuming and costly. There are certain things I simply leave to the experts and those guys are certainly among the best experts in the multi-channel eCommerce world.
I won't lie, but their software will need you to be involved. It's not just a turn on and let it run from the beginning. You need to invest your own time in ensuring everything is setup properly with help of their team. It's a robust solution that delivers results for the serious eCommerce business owner.