I have different tail packs on each of my bikes.
They're perfect for a day trip, bottle of water, puncture stuff, map, maybe a cable-lock (for locking helmet to bike). The GS one probably best all round, itos a Givi SportBag, but a slightly older version. It's got a couple of useful compartments.
None are water proof, but the Givi has an integral waterproof bag to pull over when it's raining.
They're all expandable by an extra couple of litres.
Whether it's a + or -, the Givi is hardest to take on and off. The others have clips, so can pop off in a moment, but the Givi is strapped on, so I guess harder for a passer-by to run off with...