Comic book adaptations are always a risky venture for major film studios. When done successfully, studios can create extremely lucrative franchises like Batman, Blade, Teenage Mutant Ninja Turtles, Superman, or Men In Black. The rights for Marvel Comics' Spider-Man were in litigation for years between studios squabbling over potential profits. However, these movies can also go horribly wrong. Does anyone want to remember The Punisher, Captain America, Howard the Duck, The Phantom, The Rocketeer or any of the other myriad of movie flops.
Jackman, Stewart, and McKellan are the ones who support X-Men, especially Jackman, a relative unknown in the United States. Overall, it does a good job of transferring comic book concepts onto the screen, ensuring lots of box office receipts and sequels.