My wife and I both use DA+.
We've not tried the surface air horn yet. But as another diver mentioned, even the subsurface signal is quite loud on the surface. Underwater it is a quaking noise. Most underwater signals will make a similar sound because of the precussion piston technology they use.
I will say this, my instructor has an original hammerhead that I could hear down off Lantana as far as 30' away from him. The DA+ has a more limited range of maybe 12'-15' that you can really hear it. But be warned, you do have to buzz it a couple of times, or one long buzz in order to hear it over the regulator and bubbles.
My wife and I are usually next to each other, but often just out of touch range. So we use them to get each others attention underwater. That's it.
And for that, we're happy with our purchase.
Robert
PS: If you check around some spots online, you can find it cheaper than Divers Direct - but DD will match 110% price guarantee.