I got mine a couple days ago from Amazon. Here's the short version:
Pros: GREAT audio quality, easy to use, very light, the headphone jack is a really nice touch.
Cons: The included app's sharing options do not include Dropbox, a *major* flaw in my opinion. Can't quite figure out how to make the app completely quit.
And the long version:
So, basically, the hardware is great. The software, not so much. I still haven't quite figured out how to make it work with GarageBand yet, but I'll keep trying. The included app (or rather, the free one from IK, called iRig Recorder) that you can download from the App Store to go along with it is, however, something of a mess. It records well enough - that isn't the problem. The export options, however, are clunky. Export by email requires you to register an account with IK, which is no fun. No Dropbox support is an almost unbelievable omission. In addition, there doesn't appear to be any way to actually completely quit this app short of restarting your iPhone - when you press the home button, it displays a big red banner under your menubar that shows that it's still active, and that won't go away. If there's any way to make this go away without restarting your iPhone, I haven't found it. Very annoying.
Anyhow, I still highly recommend this hardware. The sound quality is really first-rate - amazingly so for a microphone this size - and is incredibly feature-packed for a device this size. The extended neck of the device is very thoughtful for those who use cases, as I do. The styling is great. The smallness and lightness of the mic is amazing.
That said, it needs better software. I'll try getting GarageBand working with it. Or, IK, just release a better version of your own app.
EDIT: I've been trying the mic with a free iOS audio recording/editing app called Hokusai, with some good results. This may be the best app to use with the Mic Cast.