I have seen a lot of posts asking how to build a "supercomputer" or cluster with Raspberry Pis, but I have found very little with the pros and cons of actually making a network of Pis. I am interested in building my own small cluster, but I have a few questions.

Once clustered, do the RPIs act as a single virtual computer, or do you have individual control over all RPIs? Does one node have control of the others?

Is there anything that a RPI can do that is limited in a cluster? For instance, will I still have control over all individual GPIO ports?

How much faster does it make the pi? Lets say I make a 2 node cluster, over clocked to 1GHz each, will I essentially have a "dual core 1GHz" processor? Does the cluster need to be in Binary multiples? (ex. 1,2,4,8,16,32,64)

Will this complicate my web hosting running via NGINX for pi? Can the processing power of certain nodes be assigned specific tasks they handle? Such as node 1 will handle web service requests, Node 2 handles cloud storage, etc..