Application Pool Recycling

it seems that by default application pools are created with recycling in 1740 minute interval, which is 29 hours. My question is - does application pool recycling help or hinder performance. I understand that the answer may not be straightforward as I would think recycling may help with memory consumption but there may be startup/recycle cost associated with the pool. If anyone has done any testing in this area as to what would be the reasonable recycle rate, it would be much appreciated

And as a subquestion - what happens when request comes in while the pool is recycling?

thanks.