I frequently bike with my GPS unit, which was an e-trex Legend until recently, when I lost it snowboarding off-piste and replaced it with a Vista.
I use the official bike mount, angled just enough for me to see the screen. As well as being great for tracking routes, you can upload a new route from the computer and follow that. I cycled a convoluted 65km of completely unfamiliar terrain without going wrong once, all thanks to the navigation skills of my Legend.
Without a signal amplifier, you will occasionally lose reception in trees. However, the vast majority of the time it will be fine. Though there are always tricky spots, like tree filled gullies, with most tree covered it is a matter of being lucky with the satellite constellation.
You should find this image interesting:
It is of a 45km, 400m elevation diff. bike circuit I frequently cycle around Zürich. The yellow is the track as recorded by the GPS, and the white underneath is the track it put together when it was saved (filling in the gaps). Where the yellow stops, I lost signal. This occurs a little in the bottom right corner, which is heavy forest. However, the majority of the right side of the route is also under similar tree cover. Basically, I was unlucky at that point. There is also a prominent loss of signal in the centre. This was one of those infamous tree filled gullies. Nonetheless, I was still unfortunate there – this image shows the track I recorded in that section (about 5km) on another day! Interestingly, the other part where I lost signal in the top left was just old and narrow European streets.
Signal loss really is no problem. I’m sure you wouldn’t regret purchasing a GPS for biking, and you’d probably find countless other uses for it as well. You might want to consider a GPS unit with an altimeter if you’re planning on doing elevation plots. The Legend wasn’t very good at this (it tends to do massive elevation updates every quarter hour or so, resulting in huge cliffs on the plot), and though I haven’t tried the Vista on my bike yet, it’s more than proved itself on the ski slopes.