I remember using a rucsac to ride half a mile from a youth hostel to the shop and back for a few groceries, and that was enough to put me off doing it again. I just don't see why you would want a load bouncing on your back and jarring your shoulders over every bump when you can put it on the bike. And that's from someone who's used to walking thousands of miles with a rucsac.
Whilst it can get quite full it is never with anything weighty and I prefer to carry the, minimal, weight it is on me rather than the bike as it affects handling on some of the tight turns on my commute.
I too prefer a backpack, and a waistpack for lighter loads. I don't load the buke and make it leaden and unresponsive unless I absolitely have to. Properly adjusted theres no loose bouncing, and with or without one I sweat like President Biden at a pub quiz, so that makes no odds.
It would help enormously to know your budget, and the type of riding you have planned for it.
For the backpack detractors, there are some advantages to using rucksacks:
- no need to fit a pannier rack, or have a bike that has the appropriate eyelets
- a pannier has a fair amount of drag
- the obvious solution if you have more than one bike
- easier to carry around off the bike; a pannier is a rather awkward thing when you're on foot
- not bike specific, so just as useful for things such as hill walking or just walking to the shops
I used a rucksack for years commuting, usually going home the long way (18+ miles), often carrying a laptop. One thing to note is it acts as an extra layer - not so great in summer but quite welcome in colder weather. The one thing I'd advise is getting one with a mesh backsystem which allows air to circulate between the body of the pack and your back, reducing sweaty back syndrome.
I use a Lowe Alpine AirZone 35. It's not waterproof but does come with a rain cover. You'll find similar from other manufacturers such as Deuter or Berghaus.
I've had a Lowe Alpine Airzone 35 for over a decade now, they're great little rucksacks, and they still make that model. They're water resistant (good enough for showers) but also have an integral rain cover to make them fully water proof for the heavier downpours.
They don't do them in hi viz but there's a bright blue one in the current line up, there's an argument that bright colours are just as effective as hi viz anyway, and possibly more so.