I have been at ChannelAdvisor for some time now, and have enjoyed their services. Once the logic of the layout is understood, setting up auctions is fast. Listing and relisting is painless.
Like most checkout systems, there is not much use in it. eBay has made it way too easy to do it there. Trying to fight that is really not worth the hassle, as many people pay before you could possibly have time to get them to your checkout.
The store features at Channeladvisor are extremely basic, and the visual design needs help. The thumbnails are impossibly small, and the categorization system is rather rigid.
There are very flexible ways to build templates, which is good for people with the skills. The included templates are dated.
Support is relatively fast. The discussion boards are active, and the staff work on any problems with great focus. The community has been a bit touchy since the price hike earlier this year.
For the tools available, the price is decent. There is not a final value fee (percent of sales). There's adequate room for images for most sellers. They are not the appropriate choice for very high volume sellers, who could get unlimited accounts elsewhere. And, they don't meet the very cheapest prices, but the service is more stable than you'll find at Spare Dollar or other startup ventures.
Altogether, though, I would recommend them as a top consideration for medium sized sellers - those posting 250 to 3,000 auctions per month.