IF you are somewhat handy and have the proper tools to dissasemble Striders many of the centering, blade play, minor lock rock issues can be fixed! I have owned and sharpened for clients many striders(I currently own 5). Each one has its own sweet spot. Some come amazingly perfect from the factory (my new SMF S110V from true north knives for example) or you can get one on the secondary market or even from a retailer that needs minor adjusting.
I have never seen lock rock issues from brand new striders from retailers. Occasionally there may be blade play but it can be adjusted by disassembling, cleaning, and adjusting the pivot and scale screws. There is also a trick to help with centering that hinderer and curtiss have posted over on USN---this also works with Striders.
One last thing, Striders do have a breaking in period. Sometimes the pivot will be a little tight when you find the sweet spot with centering, zero blade play, etc but opening and closing will break it in so its nice and smooth. Finally, with new ones put some graphite on the blade tang to help keep the lock bar from being sticky until they break in. just my .02