The best part is I didn't even know this existed! This was something we had been looking into but couldn’t find anything out there like it with our specific needs (we are running large amounts of transactional emails).
We have used other services in similar capacities before which were not nearly cheap enough or user friendly at times - especially when dealing woth time sensitive issues where people need help immediately!! So far everything has gone smoothly using their service- very happy customer here :).