I used it before in my old tank where I had an HUGE plague. It worked alright, but my corals looked like crap for about a month afterward and my skimmer went nuts for a week.
I don't know anyone who has been able to successfully rid their tank completely just by using flatworm exit. Does it help, somewhat. Does it get rid of them all, no. And in my tank, they built up a tolerance to flatworm exit. After a few doses, it just more or less stunned them w/o actually killing most of them. Let me tell you, these things can move fast when they feel its in the tank.
If you don't care to take a risk, here's what you should do. Get a small 1/4" or 1/2" hose. Attach pantyhose to the end of the hose that is going into a bucket. Start a siphon and suck up as many as you can, while catching them in the pantyhose filter. Pour the water back in the tank and repeat as necessary. It's a pain in the butt, the hose clogs over and over, but try to get as many out as you can before treat because flatworms put of a toxin when they die, which is what fish and corals respond too. Then, if you really want to, dose your tank. I double dosed mine, let it set about 5 minutes, then did a 30% water change and kicked my skimmer back on. The following day, I did another 25% water change.
To actually fix the problem, you need to get something that eats them. Flatworm exit works, but it can cause problems. Also, these things bury themselves deep into sandbeds where the flatworm exit doesn't penetrate, which is why you need something natural to get ahead of them. Some say 6 line wrasses, which are hit or miss. A radiant wrasse is what I would recommend.
How I got completely rid of mine was, I bought all new rock and sand, took my tank to the car wash, hosed it out, put everything else back in after I treated my corals with Tropic marin dip, but still had some survive, but then got a radiant wrasse and haven't had any since last July.
Good luck!