Theory suits are a relatively new addition to the world of men's wear. They were first begun in 1997 as a cutting edge fashion label for both men and women. They definitely are focused more on the nightclub scene rather than the office or boardroom.
They are looking to combine the best of American and European styles into one brand. The fit is definitely more American while the cloth quality is from the Continent.
Theory is a little bit pricey for both their suits and jacket and pants separates. A full setup can easily run you $900 -$1000 at retail. However, they are often heavily discounted so its definitely worth a look in those cases. They are very popular with and tend to cater to a younger demographic, so their cuts are good for a slimmer build in most cases.