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Pro Powerful and versatile ORM
It requires some learning curve, but at the end it's a powerful and versatile ORM. We've been using it for a very big database in a realtime multi-frontend app with zero issues.
Pro Simple to learn
Propel uses Active Record, which is a rather simple to understand implementation of an ORM. It's also the most widely adopted implementation. In simple words: an object represents a row, you can create or edit an object to create or edit the corresponding row.
Pro Support for a large number of databases
Since it uses PDO, it supports all PDO enabled databases, with MySQL, SQLite, PostgreSQL, MSSQL, Oracle included.
Con Backward compatibiliy breaks
Since it's based on PHP 5.4+, using an older version of PHP may cause issues.