Not only is this a pleasant surprise, it's also one of the most enjoyable stand-alone listens this summer.
X-Men: First Class' score is done by Henry Jackman, a relatively new face on the scene (his biggest movies being Monsters vs. Aliens and Kick-Ass). For only having done a few blockbusters and the added pressure of writing something for an X-Men movie, this is pretty great.
The real core strength of the album is Magneto's motif, which is featured in several tracks. It's simple, yet incredibly effective. Consisting of about five different notes over two phrases, it's very pliable to different situations. Throughout the album we get it in basic long tones with string accents underneath, lower instrument accents with flowing upper strings, guitar w/ reverb, and a cool magnetic wave effect added in later. In the final track "Magneto" (easily the best on the album), it's featured in full force. Jackman's history with club music comes out here with added digital snare hits, and some of that wave-like techno background. It honestly works really well.
If the Magneto aspect of the album is great, I'd say everything else is good and solid. The theme for the X-Men favors secondary strings more than an outright melody, but the tone is absolutely right for that. The score takes a more emotional turn when dealing with Mystique and her relationship with Beast. "X-Training" is your training montage piece that features electronics more than one might think, but works well with how the movie is edited around it. Action tracks like "Rise Up to Rule" and "Let Battle Commence" are exciting, while the tracks "Cold War" and "To Beast or Not To Beast" help build suspense. And everything in between is just dandy.
Some have been upset by the use of electronics in a period piece, but it works here given the movie is clearly inspired by and paying homage to James Bond. What makes this score work and stand out from others is its reliance on themes like Magneto's or the X-Men's, over just chord changes or percussion hits. It's really the one X-Men soundtrack you can listen to repeatedly without the movie to guide you. I would highly recommend it.