I think the thing with Alias is that they reinvent the show often. Much more often than people are used to. Its the type of show that could get stale but still be popular for 2+ years but then would die. Instead they're being very proactive and never letting the actors or storylines get into familiar ruts.
This is the 3rd Season and the 3rd major storyline evolution. I'm still hooked. Alias and 24 and both incredible shows, I'm still trying to decide which one I like more. Good thing it doesn't really matter.
Also, I've said this for a long time. The pilot of Alias was one of the best written, acted, soundtracked (hehe), directed, etc... shows I have ever seen. That includes movies and TV, but most especially on TV. The series is awesome, but will never live up to that kinda pilot. I still watch it every now and then and its still exciting.