Mike Hanlon used the "Swimmers' Snorkel" for over 18 months, through regular rehabilitation from various accidents. He's a tremendous fan, although less-so of a companion product from the same company, the "Freestyle Snorkel."
The main other benefit I have found is that the Swimmers Snorkel enables one to achieve the “dead man’s float” without breathing (or being dead) for extremely long periods. After walking around with a boot with two inches of additional heel in it for nine weeks, one side of my body was completely out of whack with the other – similarly, my shoulders had tried to compensate and I was getting all manner of back and shoulder sore spots and pain. Once I was able to take the boot off 19 days ago, I quickly developed a floating and stretching routine which was both calming and relaxing to the whole body. With the snorkel in place I could stretch and relax into a completely supported floating position and bring on an almost meditative state while the body aligned itself, then do it again and again.
Another unspoken benefit: wearing the Snorkel will prevent anyone from approaching you to flirt in the pool, leaving you free to complete your training.
Swimmers Snorkel product evaluation [Gizmag]