I just watched X-Men: First Class.
I had really low expectations going in. X-Men 3, and X-Men Origins:Wolverine left a pretty bad taste in my mouth. A few odd casting choices, some rewriting of the X-Men mythos as far as characters go, and a really terrible add campaign didn't help.
Speaking as a big fan of the X-Men comics, I'll say without a moment of hesitation that it's the best X-Men movie yet. The whole 60's cold-war/Bond movie vibe works really well. There are a ton of great moments, but I don't want to ruin anything. All I can say, is that if you have any interest in X-Men at all, go see it. Even if you don't, I'd reccomend checking it out.
Also: Michael Fassbender as Magneto was pretty fantastic.