SW Motec Bags Connection make some decent tank bags that will snap onto a tank ring bolted to the filler cap, no need for magnets or straps. They even do a tank ring that can have a 12v power supply so when you snap the bag into place a lead in the bag can power your phone or sat-nav.
Lol, i knew there'd be arguments on this subject. Alot of rucksacks nowadays have a built in back protector to stop any sharp objects puncturing in the event of an off. And tbh, ur stupid to carry them if you do anyway. Last tank bag i used with magnetic flaps scratched a brand new bike tank as a bit of metal filing got attracted to the magnets and did the dirty. Never again. Definetely a rucksack for me. To me, if you have an off, then potentially it could also act in your favour by cushioning the initial impact. I use a Knox six pack, for £40-45 its been fantastic, highly recommended. It can swallow alot of stuff. I've not really used a removable tail pack/bag much, only a semi-permanent/fixed one so can't comment on that.
Depends what you are carrying, if its something hard I don't think is want to land on it in an accident, the back is not an area id like to put at risk of injury.
Here's a top tip: If you use a magnetic tank bag, go to a diving store and get some thin wetsuit neoprene (5 mil or less if poss, 3mm would be better). They'll give you an off-cut for almost nothing. Then glue same to bottom of tank bag. You'll find it makes pretty much no difference to the stickability of your tank bag, but you will no longer scuff your paintwork.
I always check my tank bag magnets to see if they have collected anything before I put it on the tank..... Most of the time, damage done by tank bags is done by dragging it off the tank, rather than lifting the magnets........and although now I rarely carry 'luggage' and therefore rarely use the tankbag, I used to use it a lot on my zxr and zzr1100....never had any scratches from using it. AL
I use a 35l Kreiga, love it ( but v expensive ) and at the other end of the spectrum, a cheap Aldi mag tank bag. Love 'em both on my 900SS and CBR1000F
