ooh...I just installed one of these--love it! I felt a little guilty spending that much money on a bike computer since my top speed average is like, 17 mph, but I love me some gadgets.

what maillotpois said is totally right--I just wanted to confirm that it does autosense your wheel size, but there's an option to configure it manually for each bike as well. There's a chart in the manual that tells you the numbers to put in based on your wheel circumference and thickness(? is that the right word). But like she says, the speed sensor on the wheel is only used if the GPS is unavailable or if you choose to turn it off for a trainer workout.

Overall, the installation/use is super easy and user-friendly. The biggest problem I had with the install was that the GPS "head" absolutely refused to see the cadence sensor. I turned the option off and on, rescanned, put the head right next to the sensor...nothing. My LBS guys were able to fix it by going into the settings for "Bike Computer 1" and selecting cadence as one of the display fields for that profile. After that, it went out and found the sensor and all was well. I guess it's a pretty common problem, so just a heads-up.

Oh, and mine came with two bike mounts as well, so all you'll need is an additional sensor to use it on two different bikes. And really, that's only if you want to measure cadence, or see distance or speed on indoor/stationary workouts.

(Also, if you need more incentive to take the plunge, you can get a $75 rebate on the Edge 305 for signing up for the Powerbar newsletter. The offer's good on purchases made from 6/1/07-11/30/07, so your friend's eligible too if she still has her receipt and original box/UPC.)