I love that it's easy to use, you can create professional looking emails for your business without having any technical skills! It is very affordable compared with other similar services out there as well - which makes them even more attractive if they are just starting up their marketing efforts or need some extra help in getting started (as opposed to needing an existing customer service team).
The only thing about principle so far has been how much time we've had since launch but everything else seems great overall!! We have seen our open rates increase significantly thanks to using this tool- especially when sending targeted messages through different channels like email listservs etc.- plus being able to track results really helps us measure ROI from campaigns/email blasts sent via Principle platform vs those done elsewhere such as MailChimp / AWeber among others. Also loved all of my colleagues at work.