I actually forked out for the Hobie sail when I bought my yak (outfitter fish) and I must say I thoroughly recommend it!
With a reasonable breeze it doubles my lazy pedal speed of 2 knots to about 4 knots. If you pedal harder it goes faster - adding 2+ knots to your peddling speed on average.
The sail is not as good as I had fantasised about however - using the sail without pedalling seems to be a bit slow (2 knots in a breeze). This is based on my outfitter fish which is a bit of a fat lady (being a tandem kayak); I'm sure the sail would perform better on the sleeker yaks.
I was hoping to be able to simply sail around Sydney Harbour while reclining enjoying a margarita, constantly pulling in fish from the lures being effortlessly trolled behind.
Oh well, at least I can dream....
As far as safety and comfort is concerned - I simply roll the sail around the mast when not in use, which adds to the yaks visibility on the water. If you want to deploy the sail it is simply a matter of pulling the main line and it comes right out. You can also pull the mast completely out while on the water and easily stow it down the side of the yak. Not much of a hassle at all.
Overall, I would recommend the sail kit.
PS: You are welcome to test my sail out on your yak if you want to join me on a Harbour fishing trip one day.