I've been using one for just under a year and won't leave home without it. I've never had a problem hearing it with my Garmin 510 no matter what the wind conditions, and I have missed hearing cars until they are right next to me before Varia. My casual rides are in the 17 to 20 mph area and I've never missed an alert. When training for TT my speed will obviously be faster, and I can still hear the alert. The excuse that you have to take your eyes off the road doesn't fly. A quick glance is all that is needed. And forget mirrors...... talk about taking your eyes off the road, sheesh!
The only time it might not work great would be if a car pulls in right behind you from an intersection or cross road. It might not alert right away. You get used to looking back if you think a car has turned in right behind you. Also, if a car slows to your pace it will appear that the car has turned off and you will not see the little dot. I've gotten used to glancing back to see if the car is still there. You still need to look back occasionally as before, but this little device has been life changing on the bike for me.