thanks for the feedback, my last question is:
is there a feedback from people who received their units? is mining using them is working as promised? is the speed as promised?
do they get bitcoins according to the current difficulty?
Yes, there's a lot of feedback on that. Many people have posted pictures, videos, reviews, etc describing their new miners. They perform as promised (well, with the exception of sucking up a lot more electricity than originally advertised in 2011). The speed is usually better than promised, and Bitcoins are mined normally, just like any other device running at the same speed.
Look in the Custom Hardware section of this forum, or browse around on Butterfly Labs' forum to find some of these feedback posts.