We've being using JoeLister for over a year now and wouldn't be able to sell our inventory on multiple platforms without it. It's pretty seamless integration and manages to auto correct inventory levels, and send out shipping orders aasap. There's no way we could sell the amount we do without using the service.
Occasionally there's a hiccup when strange addresses get through the system that JoeLister can't get fulfilled but an automated email is sent out that lets us manually correct the issue. It happens very rarely.
We've never had an issue with support and the girls and guys behind the scenes make communication and fixes super easy.
Finally for the service provided I believe the cost is right in line. Other services we looked at were much more expensive and had input dashboards that were wayyyyy confusing.