I have a client who seems to be in the same $ volume ballpark as yours. They migrated to BigCommerce a year ago and I was part of the project. The developers/designers seemed to be able to implement most of my recommendations without too many problems. Like anything there were some tweaks and a compromise or two but overall its been a great experience - the client is really happy and sales are up several hundred percent, due in no small part to how seo friendly BC is compared to their previous solution.
I also found them very receptive to some of my off-the-wall requests, not what I was expecting for sure.
