Another good reason for extending modUser.
AFAIK, that field is not indexed and since it contains a JSON string, you have to use LIKE, which can be slow and unreliable unless you're careful with the values.
With the extended class, it would just be:
$users = $modx->getCollection('extUser', array('myField' => 'someValue'));
foreach ($users as $user) {
$id = $user->get('userdata_id');
}
Extended fields are not really meant for searching or sorting. I'll be interested to see what happens with user fields in 3.0.