We currently use Amazon RDS, which doesn't support custom extensions, so pg_partman wasn’t an option for us. I haven't used pg_partman myself, but it looks to have different features.

I think an external tool is a bit friendlier for people who aren't super comfortable with installing Postgres extensions. Also, it doesn't require a restart.

I'm optimistic that Postgres will make all of this trivial at some point.

Edit: changed "more features" to "different features". One of the main goals is the ability to partition existing production tables without downtime, which seems different than pg_partman.