I think the premise of this episode was really good; the Friends making new year resolutions and trying to stick with them. Some of them were really good, while some of them seemed like they just couldn't come up with anything better.
Rachel's was good. Anything that challanges the character is good, and this one was not only challenging but well balanced. She got a few funny lines that didn't require any build up, then it worked its way into the storyline of her finding out about Monica and Chandler.
Monica's was bad. Taking pictures? What's that supposed to be good for, they didn't make any use out of it.
Phoebe's was bad. It did feel like Phoebe, but it also felt like a waste of plotspace. It would have been funnier if her resolution had had some inpact on her storyline with Joey.
Joey's was good. I like seeing him and Phoebe together, I was never a fan of the idea of them getting together romantically, but as friends they have great chemistry. And one of the funniest scenes of the episode is when they argue about her teaching methods.
Chandler's was great. It was really funny to see him fighting to control himself, and the others doing their best to tease him. Classic Friends.
Ross' was good. The bit about the pants wasn't so entertaining, but the general idea of trying something new every day was pretty good.
This episode is also where Rachel finds out about Monica and Chandler. I think it was good that someone found out at this point. Having only Joey know was a real stroke of genious; it makes the storyline work better and it's interesting that they put Joey in the role of being in the know. But that still leaves three Friends who don't know about it, and if they had all figured it out at the same time it would have been a bit too dramatic. So having Rachel find out about it at this point worked really well.
How they handled her finding out about it had some major faults though. First of all I don't see why she even freaked out at Monica calling Chandler "mister big" when it sounded very much like she was saying "mister Bing". Second she screamed loudly before hanging up the phone, even if Monica didn't hear her on the line she would have heard her from her bedroom. Plus both Monica and Chandler would have noticed that someone picked up the phone, you hear that quite clearly after all. And also, Joey and Rachel discuss it in very loud voices with Ross in the next room. I thought they were going to use that, have the scene end with Joey and Rachel leaving the apartment and Ross coming out into the living room with a shocked look on his face. But they chose just to ignore it.
Overall, funny episode but with some major plotholes.