I found a brand-new one at a thrift shop for a dollar, so I bought it (why not, right?) and have tried it out.
If you press with steam, I think it does help to sort of draw the moisture back out and make the seam crisp. But pressing with steam gives you the danger of warping the block, so I don't usually press blocks with steam. Using the clapper with no steam doesn't seem to do much.
So sort of a mixed result there.
For pressing seams, my favorite gadget is a seam strip. I think that's what it's called - basically a half-round piece of wood with a muslin cover over the top; you put it under the seam you're pressing open. I bought a set of 3 sizes and I LOVE those things. I hate pressing seams open but this makes it a lot easier.