If you are a retailer doing significant business across multiple marketplaces, ChannelAdvisor is a powerful solution. We no longer have fears of overselling and can list our full quantity. We can also manipulate our data so we can meet the requirements of each new marketplace without starting from the beginning.
We've also found ChannelAdvisor to be a good partner in helping us raise issues with eBay and Amazon and helping facilitate those discussions. The support team also goes above and beyond to help us. I can't imagine trusting our business to anyone else.
I'd also recommend that you attend the Catalyst conference sponsored by ChannelAdvisor(I think it is typically in March or April). It is a great opportunity to meet other retailers. We came back with a lot of new ideas and it was nice to sit down with experts and go through our account to make sure we were doing everything possible to be successful.