From what I've researched, if you keep them under control its not a bad thing. There are numerous posts on R2R about them if you search. I found one in my tank a month ago.
Good as long as they are under control, and not HUGE. The really big ones you need to remove. Im talking ones that are 4" or better. Nutrients kept under control limits the population. They are good because they help maintain the tank.
^^Not true at all^^ The more common Bristle Worm found in our tanks are of the harmless detritus eating variety.
The Bristleworms that cause most of the problems in our tanks are the larger carnivorous Fireworms of the family Pherecardia (Pacific), Hermodice (Caribbean), and other related species which are also effective predators, so when they get large enough they can quickly turn their jaws to your marine pets. Their body spines can inflict a painful sting so handle with caution (this is how they get their name).
If well feed it is not uncommon for Bristle/Fireworms to grow up to 24 inches in length (they sometimes appear smaller than they actually are because they are bunched together and not stretched out to their full length). Fireworms this large can turn into very nasty predators and have ravenous appetites.
They are well known to snack on Anemones, hard corals, gorgonians, Snails, Shrimp, even small fish! Large hungry Fireworms can do a lot of damage in a saltwater tank and you will need to get rid of them.
Detecting a Fireworm…
If you suspect Fireworms of attacking and gnawing on your invertebrates the only way to really tell is at night time a few hours after you have turned off the tank lights, ensure the room has been in total dark for a few hours too. The most effective way to spot them is with a red light (just secure a piece of red cellophane onto the light emitting end of your torch) searching the tank slowly with no rapid movements. Red light will not cause them to hide away like normal light does. Injured corals and anemones can also indicate you have a Fireworm problem.
I've had them in my tank for years! Keep them under control. Like mentioned in another post, remove the big ones. They sell traps out there! I made one out of PVC. I have had no problem with them hurting my corals or fish. They keep my tank clean where I can't see!