IMO they are a good way to see what changes do to the virtual numbers, much more so then a dead-on accurate way of rating an engine's horsepower... We've compared them to actual dyno results, sometimes they're high, sometimes low. The better programs that include more of the variables are of course far more accurate then some of the old ones. But when just doing the "I wonder what would happen if" scenario's on an engine that you already have a baseline on with a virtual dyno I think they do accurately portray whether what you're considering is an increase or decrease in power, and where you can expect it to show up in the rpm range....