AirPods, just like any other Bluetooth device, basically have two modes of operation. If you're just listening to audio, they use the A2DP profile, which lets one side send high-quality, stereo audio to the other, but not vice versa. The other profile, called HSP, works in both directions, but the audio quality is much lower, both for the headphones and the microphone. Therefore, as soon as you start using the AirPods' mic, your audio quality drops massively.

In fact, AirPods are slightly better than this than other headsets in some ways, if they're new enough and are connected to a compatible device, they use a slightly better codec than any other Bluetooth headset on the market, although they seem to have an obscene amount of noise reduction and other such filters.

Because of the fact that Bluetooth cuts out the higher frequencies, the effect is a lot less noticeable on male voices than on female ones, at least that's my impression. Women with higher voices, especially younger ones, are almost impossible to understand on some headsets, while some very masculine voices are almost perfectly clear.

If you desperately need a wireless headset that doesn't suck, they do exist, but they don't use Bluetooth. They are usually intended for gaming, so they offer low-latency, a much better microphone quality (although still not as good as what a wired mic can do), and work through a USB dongle. Therefore, you can only use these with a computer, not a phone. If you're going into that market, don't expect anything decent under $100 though, and you might even pay a lot more.