We moved to Sparkpost and hit a few bumps at first. However, after working through those and paying for a dedicated IP address, the service has been reliable and suits our use case. I do have the sense, however, that they need to hire more support personnel.

I'm not sure if you found it, but I believe that Sparkpost has a "public" support group on Slack. That's where I finally was able to get messages through when we ran into problems. They were fixed promptly.