All the customer service / warranty departments I've dealt with have all seemed very accommodating. I've yet to encounter one that didn't make things right, and some even go above and beyond on non-warranty issues.
KAI - had minor lock rock on a knife, and they fixed it up, re-sharpened the blade, and did a general tune-up (wiped down the liners/pivot and lubed everything back up). They'll also send small parts (pocket clips, torsion bars, etc.) at no charge, even if they aren't part of a warranty claim (eg. I've mentioned I just wanted a couple spare clips to attempt mods).
Benchmade - haven't made a warranty claim, but I've requested almost half a dozen pocket clips (replacing the default painted ones with parkerized ones), and they've just sent those along without question.
Spyderco - had a knife where the pocket clip screws were a tad too long and tore up the washers. I had taken the knife apart (voiding the warranty) to see the damage. I contacted Spyderco, informed them I had disassembled the knife, and they still sent me new washers at no charge.
Leatherman - I've sent two back to them, both broken by my own fault (broke the blade tip on a Wave, and broke the pliers on a Squirt). In both cases, they just sent me a brand new replacement tool, no questions asked.
Gerber - had an old, heavily used and abused knife that had developed significant blade play. Sent it in to see if they could tighten it back up, and they sent me a brand new knife.