DataMapper ORM v1.8.2

[eluser]WanWizard[/eluser]
Any extra layer costs performance, and an ORM is no exception.

Whether or not that is an issue, depends on your situation. My company creates (large) business applications, the hourly rate of the developers by far outweighs the extra cost of a more beefy server. If you work for free, then ymmv.

Datamapper is quite fast, the biggest overhead is instantiating all objects. So using an ORM for batch operations is a bad idea. Also, use PHP 5.4, since it's a lot faster when using objects.

Mixing AR and DM isn't a problem, DM uses AR underneath. You can even convert an AR resultset into DM objects if you want.