It depends entirely on their use, and your strategy.
While Hardcore players generally rely heavily on sound, the sound of decoys are far too obvious. As well, with no radar, you might actually confuse your own teammates.
However, if you don't use ghost, then a decoy is a nice way to add a little chaos during a spy plane. Since radar is so rare, many hardcore players literally stare at it as soon as it pops up, and don't think their actions through. You can bait the fire for a coupke easy kills if you keep moving.
You can also use decoys on tactical game modes if planned correctly. It takes a couple moments to realize the fire is a decoy, so If one team mate throws a decoy on the east side of a flag it may open an opportunity to sweep in from the west, clear the area, and steal the flag.
Even if they dont work, their fire is noisy and makes it harder to listen for footsteps and real gunfire, so even against experienced players it isnt a total waste.
In the end, it depends on the team you are playing. I usually keep one extra class on the side with decoys, and try them once or twice to see if they work. Experienced players ignore them, but it covers your real fire. Inexperienced players eat them up, and you reap serious rewards as they run madly around searching. Your milage may vary :)