it's actually very surprising how well the grout sponge rinses clean. I'm still on the original 2 that I bought and besides little nicks and tears in the sponge, you wouldn't be able to tell the difference between a new or used one.
I used to use Meg's microfiber mitts and if you want to do a little test that will sell you on the grout sponge, go take the mitt and clean one of your wiper blade then on the other blade use your grout sponge. Rinse both out in your bucket with the grit guard and you will be sold. For me, the best part is that if I'm doing multiple washes in a row, I don't have to switch to a clean mitt when I get to the next car. With a good rinse, the grout sponge is ready to go again. No need to spend all that money on mitts and the hassle of having to wash them in the washer.