Downloaded it yesterday and reading my first books on it right now. Works really well for me - painless, seamless, wireless purchase and delivery of current fiction right to the device using your existing 'one-click' settings - what's not to like??
Not sure what other folks are finding in Stanza that they don't get in Kindle App, but it works fine here. The big draw for me is that it gives me access to the Kindle store, without needing a Kindle - we already have 3 e-ink devices in the house and don't need another! If the Amazon Kindle store for your locale has the titles you want, then it's a good way of getting them onto hardware you already own.
I also must just say that I have no problem reading on the iPhone, and find the white-on-black type particularly good for reading in the evenings/in bed.
So yes, big thumbs-up from me
Cheers, Pete.