The ads say Toppik won't come off unless you use shampoo, but it doesn't take a genius to figure out that's an overstatement.
Now that I'm using it, I've just decided not to have it on if I'm going to be in water or sweating excessively. I'll wear a hat to the swimming pool and only take it off as I'm about to jump in the water. Once my hair is wet, my scalp is going to shine through no matter what. When I'm working out, I'll go to the gym with a doo rag on to conceal my dome. When I'm done sweating, I'll dry my head and put on the Toppik.
I really don't know that I'd trust any concealer not to run if exposed to a lot of moisture. That could make for some embarassing situations. Toppik has definitely made a difference in how I feel about myself, though. It might not be perfect, but I'd rather have it than not.