I agree with you the U3 is definitely a fun device. It's not much use in case you want to make money but it's definitely fun to play around with. So you still burn money, of course, but it's not as bad as with an S7. But the U3 has some issues as you can read in the thread about this device. Link:
https://bitcointalk.org/index.php?topic=827356.0In case you don't want to read through all the pages, here is the short list:
-In case of overclocking it's almost guaranteed that the device will get destroyed
-Running the device within the official specs is no guarantee that it won't get destroyed
-The Power Bricks have a tendency to burn through
-The USB-Implementation is rather flaky: So lots of connection problems are to be expected especially if you run Windows
Despite all of this, I also got two U3s and they make nice lottery-devices even though the chances for hitting a block are almost nil and it gets worse with every day. Best share so far was only 153M, so still miles away from solving a block.
You can still buy the U3 from several places. I got mine from
www.usbminer.ch and paid them with BTC but you might buy them over ebay or have a look at the marketplace here. In case you buy a used one, make sure it's actually hashing before you pay.
Hope that helps.