From what I remember, the copy protection on Eagle was conceptually simple and seemingly effective. Basically it put the license ID into all saved files. Updates to the software would include an updated blacklist which prevented loading of files from old pirated versions. Crackers wouldn't bother with fixing the load functionality as everything would work fine for the current iteration.
There were of course ways to work around it (load then save with the same version under the free license, or export the entire design as text using a ULP and then import in the new version). But on the whole, it struck me as frustrating the process just enough to encourage users who would possibly pay, to pay. (I wonder if Cadsoft made even more from unlocking design files. "You are having trouble opening that design because your fly-by-night consultant used a pirated version of Eagle. We'll happily unlock it for the cost of a deluxe license.")
There were of course ways to work around it (load then save with the same version under the free license, or export the entire design as text using a ULP and then import in the new version). But on the whole, it struck me as frustrating the process just enough to encourage users who would possibly pay, to pay. (I wonder if Cadsoft made even more from unlocking design files. "You are having trouble opening that design because your fly-by-night consultant used a pirated version of Eagle. We'll happily unlock it for the cost of a deluxe license.")