not too bad on the bars mike, not so good in a tankbag (much further a look down) I use a 'cheap' tomtom one like DDT, I have a waterproof (untested) holder as they are not waterproof like the true bike ones are.
Along with the next manouvers that are shown, they also say how far off that is so you dont have to be constantly checking, also the beauty of a satnav is that if you get the manouver wrong or take a wrong turn, the satnav just recalculates and directs from where you are.
Of course there are other issues such as keeping it charged unless you dont use it for long. I bought a little power pack dead cheap that is connected to my battery