main problem is that when walking slowly your track up might not work. usually you need 3-4mph to see the map oriented correctly.
a unit like the 60csx/76csx has a built in compass which orients the map correctly when walking...nuvis dont have that.
a 60csx/76csx with cn2008 nt and a 2gb microsd will navigate nicely for both auto / hiking use.
thats primarily what i use it for.
I definitely agree, as I have had this problem many times myself (about the walking slow thing, that is).
This is obviously a pretty big problem, but one other problem, as suspected, would certainly be blockage issues.
When using a GPS in big cities, the tall buildings will definitely effect your GPS from time to time, so yes, "blockage" will most likely be a factor.
No GPS will work perfect in these conditions, but my garmin streetpilot c340
always seems to do pretty well in big cities!