This thread is a bit old but thought I would add my 2c worth for anyone digging thru looking for info.
I currently have the Oxford Sports tail bag.
Adv:Looks
Expandable and will take a helmet. Also "expands", not "adds to" like others. So you dont have to preplan how much stuff you might want to take home with you or if stop at the shops on the way home.
Looks
Converts to backpak (but see below for caveat)
Looks
For the ER6 it sits on the seat so no scratching of paint to worry about.
Disadv:Size. Smaller than A4 so I have many things dont fit like plastic folders, bills, letters, etc. (Height is OK and expandable)
Zips. The zips absolutely suck on this and rarely work first time.. large chunky things that unless the bag is on super tight then the slack causes the zip to catch. (Did not have this problem with previous tank bag I had)
Upside down Ms Jane. For some reason the backpak straps make this as a backpak with the opening facing down! This means if you dont zip up completely (and do the velcro bit up as well) you throw it over your shoulder and listen as your stuff gets spread across the pavement!
Price.
Unsure:Raincover is a separate piece meaning I have just lost mine somewhere but the plus is you can take it off the bag to hang it up and dry it (which is probably how I lost it!

)
My previous bag (on a ZZR250) was a motordry tank bag that I think some others have mentioned. I found that great.. good size, easy to get on and off (zip works!!!) and had a rain cover physically attached (so dumbass like me can't lose it I imagine!)
I just felt it was a) a bit big and wrong shape for the ER6 tank and b) wanted something a bit more snazzy for my new bike.
In the end I went for style over substance and as you can probably all guess, that was a mistake.