Incredible! Just buy it already!
It does seem expensive for just a backpack, but it's not an ordinary backpack, and after owning it for a few months and using it almost every day as a commuter I have nothing but good to say about it! Well worth the money, and you really do get what you paid for.
When I first got it I was initially disappointed. It's incredibly basic, as there's one main compartment, a small utility compartment within, and a small exterior compartment. You can fit a bunch of stuff in there, but small parts will be swimming around and can easily get lost in the depths. I was hoping to put some small tools and stuff in it and I didn't want them just sitting in the bottom of the bag.
After giving it some thought I picked up a bunch of Pelican Micro dry-boxes. The backpack isn't fully waterproof (though it does a great job in the rain), so I wanted some dry boxes for my phone and such, but they turned out to be great organizers!
In total and calling the backpack comfortably full, here is a complete list of the stuff I had in it at one given time.
2x Pelican 1040 cases and 1x Pelican 1010 case (don't really need to specify whats in these), a pair of sunglasses, a balaclava, glove liners, a pair of soft-shell knee-pads, a jacket liner, a work shirt, a 1L bottle of soda, and an average sized sack lunch of a deli sammich, a bag of chips, and a brownie. Basically a perfect commuter loadout, now that's it's cooled off the jacket liner is being replaced with some wet weather gear and as a result I have even more room! Even better, when it's nearly empty and all cinched down it's not so big that it blows around and catches the wind.
The 25 seems like the perfect size for a commuter/day trip bag. Not so big it can't handle just a few small items, and still big enough to handle most of your needs without the augmentation of on-bike luggage.
The 15 I'd guess would be better for just packing a few things for a short ride, 20 for a compact commuter, 30 more for a day tripper bag for an extra pair of clothes included, and the 35 for a full on vacation.
Comfort is unmatched. The harness system really is amazing! When properly set up most of the weight rests on your hips and as a result rests on the motorcycle since you'll be sitting, and the chest system taking the pressure off your shoulders and putting it onto your chest means you don't loose any mobility in low speed maneuvers. It also means the pack stays put solid in high winds, combined with the cinch down straps this backpack is barely noticeable even at highway speeds in excess of 70MPH.
The design of the back is great, too. Doesn't block the exhaust vents on my jacket and the mesh kind of pads flow air decently well. You don't get the sweaty back that other backpacks will give you.
After using a Kriega I won't ever go back to a traditional backpack for the motorcycle. This one is just too comfortable!
It's a little odd when not on the bike, especially with the chest harness not clicked in, but that's to be expected with a piece of equipment specifically engineered to be ergonomic when riding.
Materials are excellent. I haven't exactly used the bag for a long period of time, but I have used it nearly every day for the last few months loading it up with everything from clothes and lunches to merchandise and bike parts. It doesn't show ANY sign of wear and barely shows signs of use! It seems to just brush clean when it gets dirty, and the material is tough. I've snagged it on stuff in the garage on the way out without so much as a scratch.
No fraying on straps, no loose strands on the stitching, it's superb! The only thing I can confidently compare it to is my CamelBak Mule, which I had for close to 7 years and seriously abused as a mountainbiker. It's been fallen on top of, thrown, snagged, dragged through rocks and brush, rolled over, basically everything short of malicious destruction. It looks the use, it's dirty and stained with a myriad of substances (blood? horse manure? mud? sweat? yes to all...), it's scratched up, and much of the elastic is worn out, but all the buckles are intact, there are no tears, and there still is no fraying or loose seams.
My Kriega reminds me 100% of my CamelBak, and I'll be happy to own it for years and years to come.
The only thing I think could be improved is an addition of some retro-reflective material on the sides. I went ahead and replaced all the zipper pulls and the side adjustment pulls with retro-reflective parachute cord and it did the trick. Even so, your jacket should have plenty of attention getters on the side so it's not really necessary.
10/10, 100/100, will buy again, completely recommended, well worth the cost. If you are on the fence on getting a Kriega, it better just be about what size to get!