they give us lockers at our place - might be worth asking if they'll get one for you? Then you can ride in whatever you like and get changed into whatever you like... suit or more casual depending what's on... bonus then is that you can wear what you like, stuff a suit on for meetings and then change back into casual stuff after... at least for some jobs
And I've never had any backpack problems... I'd only use a tankbag for longer distances... so long as you're not carrying tons of stuff in there - just remember to stick your stuff in a plastic bag inside whatever you use so it doesn't get wet if you get caugfht out by the weather!