I think Faith was a strong enough character to hold her own show. She had more layers than Buffy did. Don't get me wrong, I love Buffy as a character and a show, but Buffy was the good slayer. Comparing her to Faith, Buffy didn't have as much depth. Faith has a bad girl exterior, but she's very vulnerable inside.
And I don't think you can even compare her to Angel. Angel is making amends for Angelus' deeds, Faith was just a slayer and under no other influence when she made her choices.
So, I think a spin-off of Faith would be a good idea. But I've heard that Eliza has got 5 or 6 films in the running for 2006 🙁