Not for big sellers, Recently we experienced 2 day downtime
22 Jun 2018
Support is polite but slow to respond. Even mission critical services being down are addressed at a very slow pace. We pay over ~$12k a month for this service based on our sales volume, and in the last week we've lost money on orders and product due to their sync back to Amazon being down. Support personnel will say they are going to follow up and if they do it is not in a timely manner.
Developers are not located in the US, so support for when the service is down has a 12-24 hour delay. Again - not for high-volume US sellers.
Could anything have been better? Response time from support.