While they provide a lot in tones, their sound doesn't compare to a tube amp's and doesn't stand out in the mix.
For example, Bleeding Through is one of the bands on the second stage at this year's Ozzfest. They swear by Line 6 Vettas for their versatility. However, when I saw them play on the second stage this past July, I could barely hear a note their guitars were playing. It was especially frustrating during solo breaks. I could hear bass, drum, vocals, and even keyboards, but barely any guitar.
If you want one for the studio that's fine, but if you're trying to play live, you're better off with tubes.