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field_get_items() is the solution that is typically recommended. However, there are some caveats.

It is important to check if the field is empty. In this case field_get_items() returns false (which sucks).

$items = field_get_items('node', $node, 'field_countries', $node->language);
if (!empty($items)) {
  $item = reset($items);
  return $item['value'];
}
else {
  return NULL;
}

I think you can generally be sure that if an item exists, it will be in the zero [0] index. And isset() is a very robust thing. So the following works as well:

$items = field_get_items('node', $node, 'field_countries', $node->language);
return isset($items[0]['value']) ? $items[0]['value'] : NULL;

For some field types, the value is not in $item['value'] but in $item['tid'] or something else.
In the usual case (I can't think of an exception atm), the value will be the first thing in the $item array. So the following will also work, but this is more by coincidence than by design. So personally I would not count on it.
(If $items is a (non-empty) array, then we can safely (*) assume that every $item will be an array too, and not something else.)

$items = field_get_items('node', $node, 'field_countries', $node->language);
return isset($items[0]) ? reset($items[0]) : NULL;

The return value of field_get_items() is by-value, so it is generally ok to modify the $items array. This means you can use array_shift().

$items = field_get_items('node', $node, 'field_countries', $node->language);
return !empty($items) ? array_shift(array_shift($items)) : NULL;

(*) Most of the assumptions we make here are not based on language-level guarantees, but just on how Drupal generally works.

Btw the following trick can be helpful sometimes, but it is not a complete solution of the original question:

$items = field_get_items(..) ?: array();

This guarantees that $items is always an array. But for snippets above this does not really help us.