I have done some "testing" with expensive and cheap digital calipers. All of them are generally accurate enough- meaning, if you know what you are doing, you can consistently produce .001" accuracy with all of them.
The big differences between them have to do with:
-Feel: Not mere preference but whether the slide is smooth enough to allow you feel when you are applying the right pressure. This is the key to using these precisely. You have to apply just the right pressure.
-Battery life: All the premium models exhibit WAY better battery life than the cheapies. The right battery for all of these devices is the SR44, not the LR44. But even with the good battery, the cheap calipers struggle to last 6 month, in use or not.
-Features: SPC ports, size and legibility of the display, fluid tightness are all features that seperate the best from so-so models. Be careful about choosing a feature laden model that has features you will never require.
-Longevity: The lifespan of these tools is typically governed by the wear of working surfaces, and the buttons. The best models have carbide jaws. Parts are often available for the better models as well.
-Quality: The accuracy of the jaws is one of the most important aspects to these tools.Having the inside measuring jaws meet when the outside measuring jaws (not always the case with the cheaper models) is important if you find you use the inside (top) jaws.
I can say my trusty Chinese set (purchased from littlemachineshop.com) calibrated well throughout the range, but oddly read .0005" shy at 2.00". Not sure why or how this is possible. All of my other calipers (mitutoyo, Starrett,Tesa) read perfectly. But .0005" is within the accuracy tolerance for the tool (which, off the top of my head is +/-.001").
The key to these tools is learning how to use them properly and knowing what effects their precision.
The latest set I picked up is a Tesa powered plastic set made by Wiha. The plastic calipers aren't as accurate as the metal ones, but they have their place. I paid $15 for them on ebay. Favorite set is probably the Starrett 798 8" coolant proof. They have the nicest fit and finish. Mitutoyo, a perennial favorite, are excellent and slightly easier to read.