A nuclear weapon owned by Iran could cause some serious damage, the same way a nuclear weapon owned by the US or Israel could. Will they use it once they have it? Of course they won't. It'd be like running into the Sheriff's office and shooting down one of the employees, tell me how would that end.
I don't think Iran will use then just out of the blue but I don't really consider them rational either. So I'm not overly worried but I do think it is a monumental game changer for the middle east. If raises Iran to a whole new level of prestige and power in the middle east. One that Saudi Arabia will be very tempted to match.
Problem is what all the dominos do if that starts.