Part 1: Womenswear.
OK, let's be real here. 99% of this section is gonna be about Erik and Charles, firstly because 99% of the movie is about Erik and Charles, and secondly because most of the other dude costumes are pretty dull. I mean, Darwin's clothes were inspired by Muhammed Ali, but in the end he was just wearing a tight polo shirt and jacket. And Havok's All-American Boy ensemble could easily have been the actor's costume from that Taylor Swift video. (N.B. This is irrelevent info, but I was totally shocked to discover that Lucas Till, the actor who played Havoc, is only 22, and therefore was 18 when "You Belong With Me" was filmed. I sort of assumed he was one of those Hollywood "teenage" actors who is actually 27, like the cast of Teen Wolf.)
Like many big blockbuster movies, XMFC's strategy was to hire a really great cast to prop up some exceptionally clunky dialogue. Actually, that counts for all the X-Men movies, but XMFC wins the prize for that scene where Magneto uses his powers to kill some former Nazis. When they ask, terrified, "Who ARE you?" he replies, "Frankenstein's monster," instead of the OBVIOUS COMEBACK of, "I'm the Master Race." I'm so frustrated by this scene!!!! Like, four exclamation marks worth of frustrated! Frankenstein's monster doesn't even make sense in context, whereas Erik describing himself as the Master Race is both relevent to the situation and accurately illustrate's Erik's own view of himself and mutantkind. Aaarrgh.
All that aside, Magneto and Professor X are probably my favourite superhero movie pairing ever. Tony Stark & Pepper Potts are awesome and Thor & Loki are a tale for the ages, but the epic intensity of the Erik/Charles backstory (or in this case, frontstory) is so brilliant that it makes up for any amount of terrible dialogue. Also, they're one of the few cases where I think it's completely legit to interpret their canonical relationship as a romance, rather than it being a fanfic or subtext-based construct. My general opinion on slash fandom (if you don't know what that is, google it) is that it's awesome, but some pairings are more canonical than others. Don't get me wrong, canon "evidence" does not make any pairing more or less valid because hey, it's fanfic, do what you like. But there are a few male/male pairings where I feel like you can just watch or read the original source material and be like, "Yes, this was definitely just going on offscreen" without any need to extrapolate further. Holmes/Watson, Remus/Sirius and Magneto/Professor X all come to mind.
This is going to sound super basic, but I enjoy Erik's costumes because someone has clearly sat down and really thought about why he'd be wearing those clothes. That isn't a criticism of costume designers in general, because in a sci-fi/action blockbuster like this there are often other things to consider when choosing the more everyday costumes. Unless otherwise specified, characters tend to be dressed as some combination of "cool" and "normal", which is why you see so many movies and TV shows where every single person is wearing some combination of jeans, t-shirt, leather jacket, and plaid shirt. You want your characters to look "normal" so audiences aren't distracted (particularly if the character is a dude), but you want them to look "cool" because they're superheroes or vampires whatever. This idea is upgraded somewhat when it's a movie where everyone is meant to be very rich: then they have to look cool and expensive, but are still rather unlikely to have much in the way of individual style or eccentricity. Anyhow, Erik is a snappy dresser, and the reason for that is... Nazis.
Erik came of age in a concentration camp. You better believe that as soon as he got his hands on some of that Nazi gold, he started spending it on personal luxuries like expensive suits, haircuts, hotels, and air travel. He also has an unusually high degree of personal vanity for a guy who isn't characterised as effeminate or overpoweringly showy, which is something you don't see much in movies set in the present day. He's self-contained to the point of obsession, which ties into his self-image as a superhuman and, later, as a leader of the mutant rebellion against the inferiority of humankind. Always clean-shaven and neat, his pocket squares are folded to a perfect right-angle, and he has an outfit for every occasion. Erik Lensherr is a construct of the well-dressed, well-prepared, well-travelled 20th century man, and I'm almost certain that he got many of those attributes just from magazines and 1960s advertising, because he sure as hell doesn't socialise. If this movie hadn't come out in 2011 then I'm sure we'd see him smoking some very classy cigarettes as well.
I suspect that some viewers had kind of a WTF reaction to James McAvoy's costumes in XMFC, purely because they are so frumpy and old-professorish. Personally, I thought they worked very well, mostly because of the fantastic characterisation decision they made for Charles as a young man. Despite the fact that he's a pivotal character in all the X-Men movies, he has virtually no personality as an old man. And that's fine! In fact, it's kind of the whole point of kindly old mentor figures. But seeing Charles in this movie, it was suddenly clear how he'd matured into the Professor X we know from 40 years in the future. Arrogant and douchey, yet well-meaning: the perfect stereotype of a priveleged young genius. In order to turn into the wise, inspirational (and ethical) old Professor X, he'd have to go through some serious shit first.
Charles' old-man clothes are perfect for an old-money Oxbridge nerd, but also help to highlight the differences between himself and Erik. As opposing forces, even their mutant powers clash: mental and physical. Charles cares far less about his image than Erik, partly because he can always tell what people think of him anyway, and partly because he was born rich and has the luxury of looking like crap and getting away with it. Posh Oxbridge professors can dress in rags because everyone knows that they're posh Oxbridge professors already. Also, they're above such frivolous things as fashion. Whereas Erik wants people to respect and envy him for his appearance, and takes enormous pride in his self-control and physical abilities. We do see Charles wearing three-piece suits, but that's because it's the 1960s when that kind of thing was the norm for businesslike occasions, and they're never as sleek or trendy as Erik's.
Re: the actual X-Men uniforms, we have kind of a conundrum. On the one hand, they look heartwarmingly practical compared to the X-Men trilogy's black leather catsuits, BUT their actual creation is totally improbable. Not only are we meant to believe that Hank has multiple PhDs and the ability to be a geneticist and build Cerebro and a stealth jet by his mid-20s, but he creates an entire set of made-to-measure superhero costumes as well? And adjusts his own costume to fit his new body overnight? At least by the 2000s, the X-Men are an established team and can probably commission a professional to design their silly-looking fetish outfits. (Don't even get me started on the Days of Future Past costumes. WHO DECIDED IT WAS A GOOD IDEA TO PUT PATRICK STEWART IN BLACK PLASTIC BODY ARMOUR?)