How do you like it? I picked one up at GP Bikes last season but returned it the next week because first of all I found it to be overly large as a tank bag so I felt "crowded" on the bike with it right in front of me (made it harder to duck out of the wind too because it was so large) and, since it was a windy day when I picked it up, I found that the magnets didn't hold it on the tank very well. If it weren't for the safety straps it would've been lost on the 401.
I bought a giant hiking/camping backpack at MEC that has LOTS of compression straps. So when it's empty it compresses smaller than my old, regular, laptop bag I've had since highschool but if I expand it I think it holds 90L or something... I know I can fit 4 6 packs into it with lots of room to spare. I brought my helmet with me when I bought it to test it for size because most of their bags are very tall to hold a lot but do not expand enough to fit a helmet. Now, I can carry a spare change of clothes with me (I often spend the night at places other than home) and when I get off the bike I can fit ALL my gear into the bag so if I need to walk around with it, it's heavy but not as annoying as just carrying a 2 pc suit on my arm while also trying to carry boots, gloves, helmet, etc. The only annoying thing is that since it has so many long compression straps it's kind of a pain to tie them up so they don't blow all over the place.
I wouldn't leave my helmet on the bike for very long if I can avoid it. Someone might pee in it as was said (or a bird might crap on it or it might get full of spiders/earwigs [they seem to like bikes], might get rained in, etc). Plus, it's quite a bit of work to pull the seat and hook up the locks compared to just carrying the helmet with me or tossing it in my backpack.
