i use a mio digiwalker. i paid around 30 sovs for it from ebay which is about the going rate, i load up where i want to go, put it in my pocket and feed an ear piece through my jacket and listen to the instructions. cheap, effective and waterproof when in a plastic bag. i can't figure why people pay hundreds and festoon bikes with ugly mounting fixings which necessitate taking eyes off the road when this solution is available.
i also use it in my work van and mrs uses it when cycling places too.
down sides? it can sometimes take a few minutes to find where it is. it isn't a full postcode search, part postcode and then road name are required. the supplied ear piece was very uncomfortable so i bought a decent in-ear headphone set.
It is called personal preference
Personally I prefer to have the screen in front of me so I can have a quick glance down and see when the next turn off is on a motorway. That and I can operate it with gloves on. More than once I have hit the zoom out button to see if I can find a quick shortcut when stuck in traffic.
Picks up satellites in no more than 20 seconds (even indoors), has full postcode search, points of interest, petrol stations etc etc. Bluetooth headset capability and comes with the attachments to mount it to a bike. That is where the extra money goes..
Oh and you can probably improve your Mio Digiwalker by installing iGO software. It will install on to most of them. Might be worth looking into as has more functionality than the stock Mio software.