I prefer magnetic ones to 'strap down' ones, purely for conveniance. Good to 100mph+ (at trackdays, officer
) and the speed at which you can attach/remove them makes them good for refuling. Also, if you put your valuables in it, you can easily remove it when you stop for lunch/go into service stations, etc. An expandable one with a map cover on the top is ideal. Don't worry too much about getting really high quality waterproof ones unless you ride all year round and are going to do a lot of miles with it, the position of it it (effectively tucked in behind the screen and away from wheel fling) means it won't get as wet as panniers do.
I tend to put non-valuable stuff (clothes, sleeping bag, etc) in the tailpack/panniers and leave that on the back of the bike. I don't even bother locking them (so long as I'm not leaving the bike for more than an hour or so). I'd rather someone unzipped it, saw my dirty socks and thought better of nicking it, rather than saw it padlocked, assumed there was something valuable in it and cut the straps to get it off the bike.
I also use a tankbag for the first bagload of stuff, adding tailpack and finally panniers as the volume needed increases.