Zentail has been an amazing solution from the beginning. We began with them when they first launched and have seen nothing but improvements since. Flawless execution on all fronts. Easy-to-use, user-friendly.
Customer support is readily available, and is US based. Can't wait to see how new developments shape the software.
Personally for us we would like to see the PO module completed, FBA inbound shipments added, and the inventory forecasting module refined once those other modules are completed.
As a stand-alone multi-channel listing solution it can't be beat by anything out there.