I've had one for about 6-months now, which I got for when I was cycle commuting and couldn't use my panniers for some reason. As you say, it isn't good if overloaded, but otherwise I like it, and mainly use it for walking.
I prefer it to my berghaus freeflow 20, which is also a lot more bulky.
When walking along the flat, I have used the (very thin) hip belt fully tightened and the shoulder straps fully loosened, which gives a good air gap and helps with sweaty backs.
And the price of £22.50 is pretty attractive for something so useful.
I do have the 30 litre gourdon as well, which I wanted as a dry bag that one could use as a backpack - but that is far less useful as there are no mesh pockets or compression straps.
The 30 litre version does have the same hydration compatibility as the 20 litre sac- just no other pockets