In addition to cycling, I use GPS in my car, sailboat, and aircraft. I've never had a GPS signal failure due to cloud cover/heavy precip. As a pilot and mariner, I'd be damned scared if there was a possibility that I'd lose GPS when the weather is bad -- that's just when I need it. I've only lost signal due to overhead trees or buildings (like in the downtown area of a major city).

Now, my little Element Bolt is by far the smallest GPS I've ever used (and the least critical), but I only lose signal under trees, and that rarely.