I like that it's easy to use, with very few lines of code needed for even simple applications; if you need speed or performance, there are various techniques available to make things faster.
It is also great in terms of security - no XSS flaws were found during our audit process (and we audited over 50 projects). The documentation could be improved - they should add more examples and tutorials so people don't have to spend weeks learning how to do basic CRUD operations! We used Phalcon mainly because of its ability to handle large amounts of data quickly without any issues.