I am thinking of getting a large dry bag to act as a rucksack liner, rather unsurprisingly to keep my gear dry(-er). I like the look of the Ortlieb ones but cannot decide whether I should go for a heavyweight one, with the advantages of being robust and most definitely waterproof, or a mediumweight one with a valve that will allow me to compress my gear more effectively and yet won't be quite so durable.
I'd really value the opinions of anyone who uses these - or alternative - drybags and hear your thoughts on the merits/demerits of each. ...
You've hit more than one nail squarely on the head there.
I have several Ortlieb bags, a few cheap and roughly equivalent from Aldi or similar, and a ridiculous number of lightweight ones that I picked up cheaply when a camping business ceased trading. The Ortlieb (mine are black) and Aldi-type ones (mine are red or yellow) are smooth PVC coated fabric, the lightweight ones are some sort of proofed man-made woven fabric. Mine all close in the same way, that is flattening and then rolling up what is effectively the neck of an open-topped cylindrical bag, and then closing a clip to keep the rolled up bit from unrolling. The clipped bit then can be used as a carrying handle which is really useful as otherwise they can be a bit awkward to grab hold of if you're also carrying a lot of other kit.
The Aldi ones are a little odd as unlike the others where the neck closure is basically a flattened tube, on the Aldi ones there is a a cut-out on one 'side' and a flap. I don't think it makes for quite such a competent seal if it's fairly full so that you can't roll it quite so many times as you should. While the Ortlieb are almost all black, the Aldi ones are available in three or four bright colours which helps a lot if you have more than one as they're all completely opaque.
I've made a couple of temporary repairs on the PVC ones with duct tape and that's worked perfectly well but I don't think it would be any good on a fabric one.
Without a doubt the Ortlieb bags are the best if your only criterion is keeping out the water. But they're heavy, expensive, and because mine have no valve, as you point out you can't compress them once they're closed. So they might make a decent fender but they can be tricky to pack in a bergen. I've developed ways of packing them to minimise the air trapped. On the other hand sometimes I put a hot water bottle inside one, so I don't
want it to have an air release valve (which knowing my luck would leak anyway). Oh, and they're black.
The Aldi ones I generally use to keep wet things in so that the wet things (e.g. a parachute, quite a lot of water) don't wet the rest of my stuff.
The lightweight fabric ones see service mostly as individual day's clothing, mostly on motor cycles where they're inside another more or less waterproof bag or a hard case, and so well protected from damage. Did I mention that the Ortlieb bags are black? The fabric ones are several different colours so you can colour code your kit if you feel the need. I use them more to keep things under control than to keep things dry, but the waterproofing is a bonus which most of the time I hope I won't be needing.
