I just found this nice module and trying to integrate it to our test shop. Looks very promising!

After little wondering why module is showing a notification subscribe link on product listing page although I'm just subscribed, I made following little modification;

In the functions_general.php file on line 1167:

product_id = '" . (int)$_GET['products_id'] . "'

to

product_id = '" . $product_id . "'

After that modification link is showed as expected. Probably a bug or have I misunderstood something?