I've posted an entry on my general blog about the results I saw from a semester with Greenfoot.
While Greenfoot was not as smooth and easy to use for beginners as I could hope, it was still a very valuable tool for the class. Both it and BlueJ are excellent ways of being introduced to some of the basic concepts behind object-oriented programming. Objects, inheritance, members, etc. are all right there for you to see and interact with, which does a heck of a lot for making things clearer.
I'd like to see BlueJ's codepad come over into Greenfoot. The student I had in individual study using BlueJ and Objects First as a text got a lot out of the codepad.
I'm planning on spending more time with Greenfoot soon, to get a better idea of what could have been done to successfully implement ideas from class that didn't work out. We had our share of frustrations, but at this point I couldn't say whether they were the result of our shortcomings or opportunities for Greenfoot to improve.
Check out the post linked above. The student all got their games in a playable state and posted at the Greenfoot Gallery, which is a huge axchievement (I did not expect 100% success on that count!) They're linked, as are the students' development blogs.