I am sure the FiT gets great mileage, however I would more atribute the original poster's mileage gain to the engine braking in, and not the cold air intake and exhaust modification.
I have a 2008 Honda Fit with 30,000 miles on it. Overall, I've had a very positive experience. No defects. There are a few things inherent to the car that I do not prefer:
1. Road noise. It's loud. Fine for 1 or 2 people, but too loud to hold a conversation between the front and the back seat. (This is a feature if you're trying to put a baby to sleep!)
2. Transmission noise when shifting from R to D. I took this to the dealer, they said it's normal. It hasn't caused me any problems. I think it's just more poor sound insulation.
3. Underpowered in some circumstances. Don't expect much acceleration going up long, steep hills. It bothered me at first, now I don't notice it.
4. Interior plastic quality. Compared to my Acura TL, the interior is wearing at a much faster rate. But, it did cost half as much.
5. Low to the ground. I bottom out coming out of my garage every day. Oh well.
Overall, a very good value. I have the Sport package. I've been thrilled with the amount of interior room (and head room!) for a little car. The hatchback is very useful. Because of the hatchback and the flexible seats, I can fit WAY more in this car than a full sized car. I would highly recommend this car to someone who appreciates function. It's not luxury, but you get more than your money's worth. I tinted my windows, which sounds ghetto, but it really does look good.