Just watched Thursday's episodes and was happy to see the character Lilith back. Hopefully she'll become a regular. I love her.
She remains a recurring character (though seen fairly frequently) until Season 10, where she appears as a regular and even gets a 'character' graphic in the opening titles. Unfortunately, Bebe Neuwirth wanted to focus on Broadway so in Season 11 she's back to being a recurring character - although she still has some major storylines, which I won't spoil (although they have implications for 'Frasier'). In my opinion, Frasier and Lilith *make* the post-Diane years (along with great stuff from Norm and Cliff, of course). Sam, I really think loses a lot without Diane; he just seems to be an over-sexed, one-note character who is constantly chasing skirts.