I've not traditionally been a big proponent of ORM either - seems like too much overhead to me. However, Phalcon's ORM will be faster than my sql queries, so I use it.
With that said, in my opinion the pros far outweigh the cons. Using ORM will drastically speed up development, and greatly simplify your code. Using:
$account = Accounts::findFirst(139);
is way more succinct than:
$query = <<<SQL
SELECT
*
FROM
`accounts`
WHERE
`id` = 139
LIMIT 1
SQL;
$result = $db->execute($query);
$Account = new Account();
$Account->id = $result['id'];
$Account->plan = 'monthly'
//...etc
The downside to using ORM is it's not as completely flexible as you may always need. However, you can still use ORM for as much as you can, then fall back to PHQL when necessary.
Unless performance is super, ultra important (as in, you're handling the same number of requests as Facebook), you shouldn't need to worry about the performance of the ORM.