BlueHornet is reliable. It has very good deliverability, strong segmentation, enterprise-level reliability, and Jeff the customer support guy is fantastic. He's worked there for a long time and really knows the ins-and-outs of the platform.
The dynamic segmentation tool is well designed, much better than most other platforms that claim to have good segmentation.
I also think the message creation process is fairly straightforward and intuitive. Review collected by and hosted on G2.com.
It's old, clunky, and outdated. It has not been updated hardly at all over the past 5 years and really shows it. The WYSIWYG editor is terrible, no marketing automation (beyond basic recurring messages), expensive rendering previews via ReturnPath, clunky dynamic content, very limited reporting/analytics, a dinosaur XML-based API that is slow, no pre-built integrations, and it's extremely expensive. Review collected by and hosted on G2.com.