With all the spying going on, by all sorts of institutions and companies and in every way, right into being built into the OS itself with that piece of ... that's W10, I really think that putting "telemetry" into anything now just screams that you're making a specific point of alienating privacy-aware users or in fact any that could be described as at least somewhat advanced. Also, if you add it as opt-in, which is definitely the only acceptable way if you add it at all, then you'll be missing just the information about the average user, who just leaves the defaults and expects everything to just work. While if you add it as opt-out, you'll get that baseline info, but you'll be missing anything from most of the more advanced crowd, who'll quickly turn it off, plus that it'd be a nasty move in itself. Either way, it won't provide a valid picture to base decisions on.