i bought a lawnmower from Aquamart when it was a baby, maybe 2" max and within 3 months he is now about 5 inches and is FAT. i had a ton of GHA in my tank and he demo'd it down and keeps it gone, he even nips at parts on the glass the snails miss. On the flip side with Lawnmowers... mine eats algae any chance he gets, but my roommate got a lawnmower from Aquamart, same size i had mine, and he put his in his tank with quite literally EVERY rock covered in GHA, not even one spot was open. His lawnmower wont touch the algae... waits and would starve waiting for flakes or food. Same fish from same vendor at same size... and they do completely different actions.
sum it up... lawnmowers are nice for my tank, but they get big fast, they get aggressive easily, and they are not a guaranteed cleaner.