I use rubber stamps on my soaps all the time and they work
great (great impressions and great detail), but there are a few tricks to having it come out right.
First, the stamp's design should be fairly simple and well defined, i.e. not too 'busy' or bogged down with lots of tiny, crowded, complicated details.
Second, the soap needs to be stamped while the soap is fresh from the mold, or at least still fairly dentable to the touch. I stamp mine as soon as I unmold and cut. At this point I should probably mention that these freshly unmolded soaps that I am stamping right away are
gelled soaps. If ungelled, I would wait a little longer until they have firmed up a little bit (but not so long that they become too hard).
Here are some samples of my soaps using rubber stamps (all were stamped with rubber stamps from Michael's craft store except for the bee stamp and the thistle stamp, which are Milky Way soap stamps):
http://members.cox.net/ssfkjfalf/AllMySoap/Start.htm
My favorite rubber stamps to use are the sun stamp, the star stamp, the butterfly stamp, the leaf stamp,the lizard stamp, and the dragonfly stamp. Those all work great and are good examples of the kind of stamps that are not too 'busy'. In the pics you will also see a lighthouse stamp (second or third soap down). That is a good example of a stamp that is so 'busy' you can hardly discern what it is.
IrishLass