What do you like best?
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.
What do you dislike?
It's old, clunky, and outdated.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.
Recommendations to others considering the product
Just give it a good test. Make sure you haven't been sold features that are terribly difficult to use. If you are a batch and blast sender, then BlueHornet might be a good fit. If you are looking for marketing automation, go elsewhere.
What business problems are you solving with the product? What benefits have you realized?
Going back to my original comment, we use BlueHornet when we want something reliable. We've used it for 5+ years, and we know where it's lacking, but we also know that it won't go down, that the segmentation tool is very flexible, and that the deliverability is good.