I have found aluminum/carbon bikes to hit my sweet spot with regards to cost, liveliness, weight, and rideability/comfort. Aluminum frames are cheap. Made of good tubing (e.g. Easton Ultralite, CAAD 7/8, Bianchi SL), they can be made to reduce the harshness of the ride. Carbon seatstays take the last bit of the edge off. With a carbon fork and carbon seatstays, an aluminum/carbon frameset offers a good ride without being what some call "dead."