are sponges good or bad?

I have a couple neat colored sponges growing on live rock and vining out,my question is-Are they harmful?helpful?neither?take over my tank and smother everything?

 

Helpful. They are filter feeders. I have never seen them get out of control and they are pretty easy to prune if they ever try it.

 

They are good for your tank, they can be difficult to grow. They filter out and feed on organics in your tank. Some have algae associated with them like your corals and need to photosynthesize. If they are growing I would leave them be. They can produce chemicals when in the vacinity of othe sponges and corals although I have got quite a lot of sponges growing next to corals and both corals and sponges seem to be growing fine.

I too have never seen them get out of control.

Some of the most commonly seen sponges are pineapple sponges, white golf ball sponges, orange or brown boring sponges.

 

thanks and K+ to both of you,i thought they were good but wasn't sure,I have a yellow one incrusting a peice of liverock with probably 150 to 200 small brown feather dusters growing out of it,the sponge is spreading pretty good,but its in a place where it has room to grow,but it's starting to get really cool looking and i didn't want to get rid of it.Also have a blueish one thats starting to get a little bigger(still small)but the color is awesome!