I purchased a soap stamp off of etsy a couple of months ago and just had the opportunity to use it. It works great!
I found out right away that there is a trick to stamping soap. I was hoping I could stamp all the soap I already have cured. Although this may be possible, it didn't work well for me. The stamp barely made an indentation in the soap and was unreadable.
It was only after I made a new batch of soap and tried the stamp again was I able to discover the trick. Your soap should be cured about 2 weeks in order for the stamp to best work. This allows the soap to be hard enough that you don't press into mush but yet still soft enough that the stamp makes a nice, uniform indentation.
I'm pretty happy with the stamp and the etsy shop I bought it from.
**Update** I just found this excellent tip on the Bramble Berry website! If you have problems with the soap sticking to your stamp, try placing a sheet of plastic wrap over your soap before you stamp.