I don't think any of Garmin's sports computers are WAAS enabled, which means none of them are going to be able to receive much in the city. I don't know about other brands, but I suspect it's the same thing.
Satellite reception is line of sight. If there's no WAAS and no view of the sky (like in the mountains), then no GPS will work.
I've never had trouble with clouds, and the newer "high-sensitivity" receivers (I'm pretty sure the 305 has the same receiver as my 705) do quite well under porous cover like wet trees. I do have trouble with power line interference ... that's where the cadence/wheel sensor comes in handy.
Speed comes from what you put behind you. - Judi Ketteler