Well, I was lucky enough to be in Dubai last week, although it rained almost the entire time.
I got a bit lost walking from the office to the airport (I have no sense of direction whatsoever), so I whipped out GMM and tried my luck.
The maps are incredibly detailed, which is great, but in a 20 minute walk, I didn't get a GPS lock at all. Thankfully, the cell tower id is pretty decent, and Nagi (a righteous dude in marketing) had drawn me a map. I scoffed initially, but thanks to him, I got me home ok.
At a couple of points, certainly, the high rise buildings would have prevented the lock, but I was out in the open quite a lot, too.
I don't really know much about GPS, beyond it using satellites to triangulate you, so it's possible there are fewer over Dubai, or they were low in the sky or something, but it was pretty annoying.
I might have to do some Christmas reading.