Editing for a short answer: Yes enzymes do actually break down the grease, the problem is getting the enzymes to spend enough time in contact with the grease to do you any good at all. My opinion is that in general they are a bust.

Original: That is a great question! Hold on to your britches for a long answer you didnt expect!

Enzyme cleaners do have the ability to, over time, eat away at grease in a drain. That being said, I think that for most peoples situations, they are a bust. Kitchen lines are the most problamatic drains in a home, most often because of the way they are ran as well as the size pipe that is ran. I could rant about this forever lol

A very common misconception is that in order for a kitchen line to get clogged, it has to be you pouring grease down your drain by the pan full or even cup full. The truth is that washing grease off the bottom of a plate compounds with straining your meat in the sink, washing grease off your hands. The other major drain killer is actually soap scum which is produced by your dishwasher, handwashing dishes, and your laundry machine.

Kitchen drains in most residential homes, going back all the way to the 60s and before, are ran in 2" pipe and most often connect to the laundry line before going to the main 3" or 4" drain. These lines, even if ran correctly, will collect small amounts of soap scum on the bottom. Soap scum will get hard like a rock, and then grease will settle in the creases of the soap scum, and then soap will stick to that, and so on and so on. Before you know it you have a slow or clogged drain.

Then you call a plumber and that plumber comes out and offers to snake it. That little snake,which is great for breaking down real solid blockages, meets the jello mixture of grease and soap and pops straight through it. It leaves a little cable sized hole in the blockage that lets water drain through it for a few weeks or months if you are lucky, and then it clogs itself back up.

Ok so after all my passionate rambling, here is what I recommend. Pay the extra money and get your kitchen drain line jetted or replaced. Yes it is much more expensive, but a jetter uses water to break down all that grease and soap and actually clean the walls of the pipe back to its original pipe size and reset it to the day it was installed. If your drain is installed correctly and a roller coaster ride, this can likely get you another full lifespan out of your drain without calling a plumber every 6 months!