From a design perspective, recessed fixtures are great, because they brighten a room without cluttering the ceiling. But from an energy efficiency perspective, they're duds. Because these fixtures usually aren't sealed and can't have insulation above them, they allow heated air to escape into the attic.
You can replace them with newer, airtight models, but that can get expensive. There's a far easier fix: Buy a retrofit kit that screws into the existing fixture. The best kind seals around the rim and behind the bulb, converting your old fixture to one that is airtight and insulation-rated. You'll need a special fluorescent bulb (4 pins, 20 watts), but given the average expected life of 10,000 hours, you won't have to buy a replacement anytime soon.
TCP Dimmable Retrofit Kit: $41.30
TCP TCX Dimmable Springlamp: $6.50