I've got them on both my road bikes (Secteur and Tricross) and I rate them highly. Once fitted, you can pretty much forget they are there as, unlike bar mirrors, they don't add to the width of the bike and they don't stop you resting the bike against walls etc. Mine haven't been hit or touched since I fitted them in March.
Potsy is right that a pannier or a large saddle-bag may obscure the view but I've adjusted my Bike-Eye to minimise the impact.
As they look under the saddle and between your own legs, there isn't a very wide field of view, it's pretty much just stuff that is straight behind you, so they do not substitute for a shoulder check. Then again, whether in a car or on a (motor)bike, I never rely on any mirror alone and I always do a shoulder check too. I love the Bike-Eye for giving me that at-a-glance image of the road directly behind you, a view which can be pretty tough to see by turning your head depending on the type of bike you are riding and personal mobility.
The Bike-Eye is, as I recently discovered, also useful when riding in a group as you can easily see who is behind and how close they are without the potential dangers of turning in the saddle.