I'll second Torg. The rules system itself is fascinating...lots of fun new rules with drama decks and all of that (though the rules are not as completely 'universal' as it was promoted). The setting though was the real gem...and the way the different settings and the rules interacted even more so.
I also loved alternity...though, in my case I was fortunate to get my fix from a nice little campaign. Still have all the books stored away. As much as I love Alternity, I am, paradoxically, working on a home conversion of the Alternity (and particulary the STar*Drive Setting) conventions to the Shadowrun 4e rules...which well deserved the ENies they received.