From what I understand the user name replaces the page id number in the URL.
Instead of facebook.com/544654368763541 it can be facebook.com/myproductisgreat
IDK about SEO potential, but branding potential, it's worth the few seconds it takes to make.
But beware, if you don't have a username for your personal profile first, facebook will make your userprofile the name you try to use on your product page! So, go to your personal profile and make a username for your profile. I suggest reading more about this before you take action.
Better yet, make a business page instead of a fan page under a profile, then get people to like that and create the vanity url. In the long haul that is what you will really need.
Oh, and yes it is good for SEO
Ben Fisher has over 15 years experience in entertainment and technology Internet Marketing. Mr. Fisher is an experienced veteran in the SEO and Social Media space. Ben has worked with such clients as Seagate, Hostway, ValueWeb, Godaddy, Ipower, SoftLayer, CodeMasters, Microsoft and D3 Publishers. Connect with Ben on Google Plus
In initial state every Facebook page need 25 likes at least to get specific available user name, user name is the word which will give you a specific identity when people search you in search engine because it will add in your URL in last and we know that everyone wants to his or her URL decided by him but it also depend whether that user name available or not otherwise you have to find other which may be so close to your desire user name. once you will find user name it will always add in your URl for forever.