I have the RF 23U and really like it. Yes, it's on the expensive side. If you're an accountant living on your numpad, definitely worth it. I don't use mine that much, but still happy I bought it. I had an Adesso before the 23U that I returned. Didn't like the layout. Note the position of the -, +, =, / keys does vary by manufacturer. Numpads are not standardized like keyboards, so check that out before you buy.
Here's a review comparing the 23U and a Filco. He's the most boring guy you'll ever listen to and you'll want to beat him with a bat before it's over. But at least listen from 5:15 where he talks about n-key rollover. And then hit him with a bat.