I have two a small Hydroback
and a classic 3l - i use this for the longer sessions - it has a little pocket that you can slip your keys into too.
Is it annoying to wear or no worries?
You dont really notice it so long as the flappy bits are either tied up or tucked away.
They are really handy things to have especially if your into other things like Cycling, long walks etc etc etc etc. even working round the house/garden where you dont want to stop what your doing to go and get a drink.