ZT/Kershaw has sent me replacement parts (clips and screws) over the years without any issues. They don't usually reply, but a few weeks, maybe a month later, I'll have the requested parts in the mail. I've never sent them a knife for sharpening or repairs, so I can't tell you how well they fair in that department (though reviews from others vary widely). The did once send me a replacement spring for a Kershaw Dimension which wasn't the exact piece I needed, but it still fit the model so I didn't pursue it further.
CKF has also sent me parts, and their communication was very prompt and friendly. Unlike ZT, they charged me for the shipping, which is fine by me.
WE knives sent me a replacement screw for one that stripped. Like CKF, their response was timely and polite. I didn't have to pay a dime, and they included a free WE-branded cap (too small for my head, but I appreciate the gesture).
And last (least recent manufacturer I've dealt with), Reate has helped me out with a number of issues. Some examples: I had a Reate Future that was sent to them for fixing at the end of 2017; it got lost in the mail, but they maintained communication with me and kept up the search, and a few months later they found the package in their local postal system then promptly repaired and returned it. They've also sent me a part for a midtech they manufactured as an OEM, the Sean Kendrick/David Mosier Hybrid Moment, which was a few years old at the time. Reate will take care of you, even if it takes a while for them to get the repairs done or send replacements.
I may be missing out some other manufacturers, but these are the experiences that come to mind (excluding custom makers and the like).