The Avalon Quartet played at the MSM last week, and I finally edited together this little video. Feel free to enjoy it.
video: Avalon reaction
I thought Greg's comments were apt: it's a different experience going to the symphony than a string quartet. For me, it highlights what makes live music special in an age of automated performers and mp3s: it's about having a communal human experience through music.
At the Symphony, there is something for everybody: a billion instruments to fall in love with and a conductor who ties it all together. But most of the pressure and attention is on the conductor; if you don't like him (or her), the ship is sunk.
But a string quartet is more about the four individuals. This spreads out the focus more evenly (but still I'd say Violin I gets the most, then Cello, then Viola, then Violin II) but ends up requiring more dynamism from the performers themselves.