Favourite twin tips? I have the original Salomon Pocket Rocket for powder and spring slush, Dynastar Troublemaker for everything else, Rossignol Scratch Sprayer FS for my "rock" ski, and a brand new set of '06 Dynastar Big Troubles that I'm itching to try out. Don't really have a favourite as each ski has it's own unique feel and characteristics.
Demo if at all possible. When demoing, ask the ski tech if the skis have been recently tuned. My son and I demoed the Head Mojo 80 and 90, which we expected to love due to all the rave reviews, but found we didn't like them at all. I have heard similar complaints from other skiers. It seems that maintaining a proper tune on demo skis is often overlooked. Dull edges, deep scratches in the base, and no wax will make those fancy new skis feel more like 2x4's.