I don't have any trouble at all with the zippers on my 3 backpacks, but they're all Ospreys so that might make a difference. They all get a lot of abuse.
Just as an aside since we're talking about zippers and backpacks, if you ever want a truly amazing zipper experience out of a pack, get a GORUCK GR1 (or any GORUCK, really).
These are built to tolerate being used for rucking with 100 pounds or more of weight plates inside. So the zippers are tough, but ultra-smooth as well, and the metal zipper pulls have been removed and replaced with loops of para cord that are shrinkwrapped with rubber tubing. Easy to grab even with gloves on.
Also the best backpack in the world for carrying a laptop safely and easily.
If we're pitching our favorite bags, I'm going to throw in DEFY bags because they're made in Chicago up the street from my house. I've owned one for 5-6 years now and it's been absolutely bulletproof carrying all my shit all over the world.
I've been buying REI backpacks, they tend to grossly oversize the zippers, particularly on their travel backpacks. I have not had a zipper failure in, well since 2008 which was the last time I bought a backpack, because they (50 liter "large" travel backpack, and 18 liter "flash 18" backpack) haven't failed yet. I did have a clip failure on a timbuk2 messenger bag, but they gave me a replacement for free at one of their retail locations.
The only backpack that gave me zipper problems was so cheap it fell apart within one minute of use. The one I got afterwards was on the expensive side but it's now ~16 years old and has survived daily use pretty well, the zippers always worked perfectly despite me having put them through quite some strain.
When a zipper fails for me it's 100% a jacket. Cheap, expensive, doesn't matter, they all get stuck now and then.
That's often so with me. I've an ex-NATO military jacket that I bought from disposals. It was new old/surplus stock so I am the first to wear it.
It's by far my favorite jacket and it gets a great deal of use as it's not only excellently designed (in that everything about it is immensely practical), and that the quality of the materials used in its manufacture are excellent (it would be difficult to find better). Originally the military must have paid top dollar for them (and it's much better than the one my own country's military issued me with decades earlier).
Nevertheless its zipper is a perennial problem in that it's hard to engage when I'm in a hurry and it often jams or is sticky at the start end. Fortunately, it has a large flap that covers the zipper from top to bottom and it is secured with a vertical row of buttons!
I like that, there's nothing like a bit of 'bootstrapping' to guarantee things will always work.
I have at least four good backpacks, one used for hiking, another for short trips and the others to store my laptops, cables and related paraphernalia.
You're correct, the zippers take a considerable punishment due to their shape and the fact that I often squeeze too much stuff into the backpacks. It's usually a strain to do them up.
I don't often have zipper problems but it's always on a backpack, probably due to the abuse they take.