When I've gotten friend from a job they stopped being mentally classified as a coworkers and started being classified as a friend I met at work. Aside from random bullshitting about shared experiences or whatever, I rarely felt compelled to discuss work with them any more than I did with other friends, and they can be really great at giving frank feedback when other coworkers might not have the guts. Unless there's a boss/subordinate relationship or a romantic connection, I've never seen or experienced any real problems or drama spring from workplace friendships.

I don't even have that many real 'friends' (non-colleagues/family)... and I still agree. Separate is best - even if that means one side suffers. Solitude beats being taken advantage of, every time.