user api

Im making a strategy game (not in Drupal, only PHP with Javascript, you can see in in here www.iasoftgroup.com/xeno). For the login system I use a simple user login/register module. It doesn't do a lot, just the basic data sanitation, so it ocurrs to me that maybe i can use the Drupal API to make it work for me.

So I wanna know similars experiences and ideas of how this could be done. If it easy to do it?. Are there any problem or pitfall that we should be aware of?