I was just about ready to apply for a visa and having spoken to various members, originally I had to apply for a visa if i wanted to stay six months or less now according to new rules on January 8th or it could have been June 8th, 2016 its either one, I can go to England for up to six months without a visa.
That's not a new rule - US citizens have been able to come to the UK without a visa for up to 6 months for years now.
However, if you have any previous refused visas or entries to the UK, or any criminal convictions, you should apply for a visa in advance.
Also, if you are coming to visit for more than a couple of months or so, it might be a good idea to apply for a visa in advance, since the longer you plan to stay, the more scrutiny you will be under from UK immigration, so applying before you travel will allow you to know you have permission to enter the UK before you get on the plane. If you plan to stay for 6 months, they will be looking to see how you can afford to stay for so long, why you have 6 months available to visit the UK (i.e. i.e. most people can't take that much time off work), and that you have ties to the US that mean you will definitely return at the end of the 6 months and won't attempt to stay in the UK for longer (such as a job, a home, family to get back to, appointments, events etc.).