Why a ScanGuage?
 

OK, I'm new to EM. I see everyone talking about their ScanGuage's. Why get one? What info does it provide that is so valuable that it justifies its pricetag?

the main thing is it provides you with real time MPG, which can be invaluable in adjusting your drive style.

also records and calculates all kind of trip data and works as a code reader

ScanGaugeII - Trip Computers + Digital Gauges + Scan Tool

I love my Scangauge. It will easily pay for itself. You can really see exactly how efficient your driving is and also how to most effieciently drive your car. I use the TPS (throttle position) readout alot. If I want to drive about 35mph, I will try to hold the throttle so I get a TPS of 13. 40-45mph usually a TPS of 14-15. Or a TPS of 15-16 for 50-55mph. The steadier I hold the throttle, the better my gas mileage usually. I've learned little things like how much gas the headlights use, or the difference in gas usage of sitting at a light in neutral or in drive. Lots of little things like that. My FE keeps improving because of the scangauge.

I'd like to see a Scanguage Light... something with just the "mileage" features, at a cost of $50

The lights use gas...
so I guess the CD player does too?

Everything that puts a load on the engine will undoubtedly use some gas, but it depends on how much energy is required. I barely notice any drops in FE when I use my radio but if I used it for a longer period of time and had more energy draining apps connected the effects would be more pronounced.

Just ran some quickie numbers:

annual drive = 12,000miles

without SGII = 38mpg
with SGII = 44mpg

assuming gas costs $4, then without SGII I spend $1263.16 on gas per year.
With the SGII, I spend $1090.91 per year or I save $172.25 annually which pays for itself. Obviously these are only rough estimations and I currently drive better than 44 mpg and the reg gas here costs about $4.35 last night.