I’ve got a wee camelbak runner thing, space for a bottle (though it doesn’t like big ones), a multitool, a couple of packs of emergency haribo- I like just being able to grab it and go. I had to add an extra strap to keep bottles in as it’s really not designed for mountain bikes, but it works pretty well. I can squeeze in a skinny tube and a co2 can but I don’t, generally, it gets a bit bulgey.
Basically, it’s alright, it’s not bad and it didn’t come with any Enduro Tax. But I’m not totally convinced, for anything longer than a quick blast I want more stuff than it can carry, and I can’t see any reason to use a bigger one instead of a camelbak.