It depends on the device in question. iPhones use a heat sensitive touchscreen and will not operate with a gloved finger. Try pressing your iPhone screen with a pencil or similar object and you will see that it doesn't work.

On the other hand, a Garmin 800 uses a pressure sensitive touchscreen and will work fine with gloves on; albeit you will lose a bit of dexterity but it will still work..

No.

I have used these gloves with a garmin dakota gps, and they work fine.

I can switch screens, but can not do too much, just too bulky.

I have not managed to get the fingertips to work with my iPhone 3Gs.

I've been able to use them with my iPhone when wet (due to sweat or rain) but you have to press firmly.

My hand measures 21cm longest part and 11cm widest part. The large size fits me snugly.

I take a 7.5 glove size and I bought the large. They are quite snug and would not take a liner.
My hands are fairly average for a bloke.

My hands are large to XL, these gloves run one size too small.

Prob go 1 size up, I haven't got big hands but I wear a Large.