On average we stay 160-170 days south inside the US and have done so for many years. I just filled out my 2014 W8840 form this morning, one for me and one for the DW. It takes about 5 minutes each and I make a copy for myself and keep it with the passports.
I was asked once if I was aware I had to file it and told the US customs man I had my latest copy. In all the years I've filed them I have never received a reply from IRS or even an acknowledgement they received it.
With the new cross checking coming into effect you had better file one as they will know exactly how many days each year you spent south. You also have to include any mid season days or cross border visits.
As for the changes it may or may not include the IRS changing but for them it is 182 days max or if over you will become taxable.
