The degree to which it's effective seemed to me to be a case-by-case thing. While working there I tended to suspect that Cyc people underestimated the degree to which you could get a large fraction of their results using something like Datomic and it was an open question (to me at least) whether the extra 10% or whatever was worth how much massively more complicated it is to work with Cyc. I might be wrong though, I kind of isolated myself from working directly with customers.
One issue is just that "useful" always invites the question "useful to whom?"
Part of the tension of the company was a distinction between their long term project and the work that they did to pay the bills. The long term goal was something like, to eventually accumulate enough knowledge to create something that could be the basis for a human-ish AI. Whether that's useful, or their approach to it was useful, is a matter for another comment. But let's just say, businesses rarely show up wanting to pay you for doing that directly, so part of the business model is just finding particular problems that they were good at (lots of data, lots of basic inference required using common sense knowledge) that other companies weren't prepared to do. Some clients found Cyc enormously useful in that regard, others were frustrated by the complexity of the system.