I'm in one currently, and right now the biggest con is that you can be with the person physically (not meaning sex, but just that you cant be there to hold them or hug them or anything).
But as long as you care for each other and are willing to, it can work. I mean, in today's day and age of cellphones, webcams, facebook, emails, and (if all else fails, or you're just old fashioned like me) letters, its easy to keep a relatinship going as long as you find the time for each other.