It's nice - but I find the BitcoinWisdom mining calculator much better still.  Yours only calculates your earnings after the next difficulty increase.  In reality, a miner needs to be able to model their returns after each difficulty increase.  Without taking that into consideration, a miner will have an over optimistic expectation of return.  Also - you should factor in the initial cost of hardware as well.  With a couple more updates, I think this would be great!