As for the elevation, it has little to do with weather. It's not calculated using any kind of barometric pressure or anything. It's all triangulation using satellites, and it's not the most accurate method. It's really helpful to see trends, but I wouldn't trust absolute values.

The speed is USUALLY calculated using the GPS, but if you buy the cadence sensor, it also has a spoke magnet/sensor, so if you train indoors or ride in a tunnel or something, it still is spot on. On my first ride with the whole system, it beeped at me at about 10 minutes and said "Automatic Wheel Size Detected" or something like that.

Overall, I love the system. I'm actually about to write a review of it, because I have learned a few things that I didn't know before I bought it. I would recommend it to anyone. I actually have the Forerunner 305 because I do triathlons, and then I have the quick release bike mount. It's perfect. I'd recommend this to anyone that does multisport, or even to anyone that enjoys running once in a while.

BlueJay mentioned the lack of Cateye's above and below arrows to tell you how you compare to your averages, but with the garmin you can program speed/heart rate/cadence zones, and it will beep at you if you go outside your desired range. It's not as automatic as the Cateye, but it's doable. It also has a vitrual partner that you can race against and it will tell you how you're doing, or you can race against past rides/runs that you've done.

The only thing that isn't easily viewable is a total odometer, but you can calculate it without too much trouble on your own, but that is definitely an oversight. Respond if you have any other questions, or you can even PM me, because I've tinkered with mine quite a bit in addition to training with it.