I've done a lot of work with PTC fuses. In general, they are not fast, but I would say they are comparable to a "slo-blo" automotive fuse. There are a lot of types and a lot of curves, I generally use the Littelfuse ones. Detailed curves and specs on their web site. Curves vary with voltage rating.
In my opinion they are fine for protecting conductors, or something like a solenoid or transformer or motor where I2T is what counts. I usually size them pretty small, there is a lot of space between the trip point and the hold point, and you can run them at or a little above the hold point without trouble.
Now, what you get with "no-name" products is a different topic completely, but most of my ebay purchases have included traceable manufacturers markings.
FWIW, I use a .5A hold, 1A trip PTC fuse in series with the feed to my Fastrack uncouplers, to protect against "stuck buttons." This gives me pretty much full current for about 5 seconds, tapering to a very low value about 1 second after that. Uncoupler draws about 1.8A, if I recall correctly.
