There's a difference between those who join to make a difference in their lives & those who join in the new year because they've made a resolution to "get healthy" or "lose weight". Those who genuinely want to change their lifestyle by going to the gym generally don't wait for the mass signups of the first week of January, they join up through the year because to them it's not about an arbitrary date to make the change, it's about making the change regardless as to if it's the third Wednesday in September or the third of January. I have nothing bad to say about those people because well, I've been the overweight guy who decided enough was enough, they're the ones who become the guys you speak to in the changing rooms fairly quickly.
However the "resolutioners" as I call them you can spot a mile away, they're the ones who kid themselves on every year that it's going to be THIS year they're going to do something about their fitness, they rock up the first Monday of the New Year in brand new gear & so full of optimism, they'll sign up for the whole package (classes & all that). Problem is as several have pointed out, they barely stick it out, there's the odd one who'll prove to be made of sterner stuff, but 90-95% of them will not be seen in the gym for the vast majority of the year - that's not being a dick about it, that's just telling the story the way it actually is, I don't dislike them for it, I actually feel sorry for them because they're missing out. (And feel especially sorry for them given they'll be paying a decent chunk of change to miss out on the satisfaction that a good session will give you)
The other thing that kinda bums me out about it is the fact you'll probably see the self same person again the next year doing the same thing all over again with the new kit & the same old good intentions which die out after a few weeks. If your average gym worker had a pound for every time they've seen a "resolutioner" they'd be able to retire before they served 10 years in the role. If that sounds judgemental then I do appologise, but ultimately there's no real way to sugar coat the truth in this situation, I'd just rather they'd make the change, personally I don't mind if it's busier as a result because it means they're doing more than paying lip service to it.