We like that we can select the different sending volumes and schedules based on our unique needs.
Their customer support is responsive.
Overall we've had a great experiece with 1.5K+ mailboxes connect to warmup inox.
It's easy to setup and it makes a difference for being reported as spam, before we start using a server, we always use warmupinbox so that when our clients are being added to the server their e-mail doesn't go to spam.