I read on my touch not an iPad (yet) so I'm not sure if this will help.
What I found is that, if you read through Stanza, you just have to click on "add to library" rather than download a sample. If you haven't bought it, what appears in your library is the sample.
As for the quality, yeah, there are both well published authors and complete clueless newbies on Smashwords. But you can often sort out the worst writers by how bad the book description is, and the rest you can sort out with the samples.