It has a really good transactional API and the dynamic template is nice and easy to use. Ruby gem for SendGrid is good as well.
What I love the most about, Sendgrid emails is that they are not spammed when they reach their target's mail box and are always marked as important.
The built in tracking is quite nice and they make it very easy to configure. Knowing not only opens/clicks but unique values was great.
I tried for 2 weeks to get in touch with them (trouble tickets, emails, and phone calls) and never got a reply to anything. This is a TERRIBLE company to do business with.
Service is terrible lodged a customer service request replied quickly with useless information then didn't hear from them for 4 days.
When complaining to Sendgrid I spoke with several people who were empathic to my frustrations about the program no longer working as it should.