This case definitely does the job.
It has a nice hard(ish) shell and a useful ribbon to pull the Circuit out. It also has some extra space on the sides which I use to keep a set of spare earbuds.
The lid opens upwards, making it very easy and convenient to pull the case out on the bus, put it on your lap, and play the Circuit... then shut it back down when somebody sits next to you and gives you the look (speaking from experience here).
The one minor complaint I have is the padding on the lid. It comes with a piece of pyramid-shaped foam that applies pressure to the unit, preventing it from rattling inside. This is great, but the foam is the same thickness across, meaning it pushes harder on the knobs than the rest of the unit. If it was more shallow on the top where the knobs are, it really would have been the perfect case.
Minor complaint aside, this accessory is essential if you're an outdoors/train/bus Circuit user, which is one of the best uses of the machine.
Highly recommended!