I have one and use it frequently - the results are generally excellent, but it does have issues in low light. They're expensive, and there are things which, for most people I think are a lot more useful, so unless you have spare money laying around (In which case let me know and I'll PM you my PayPal info) I'm not sure that I'd recommend it.
I was considering getting one for Sports photography, specifically Hockey since Hockey rinks are poorly lit. Are the before/after pictures on the website just for show?
Define for show. Most of the photos in the after gallery have a warm tinge to them. The expo disc is just a tool for white balancing. There's nothing this thing can do that you can't just do in Lightroom. It just gives an automated reference for your images, nothing more. Personally I prefer a simple grey card. You can get small ones that fit on your keyring, or ones that come with colour checker charts. But just remember that this is not a magic device.
Your best bet will be to shoot in RAW and fine tune the WB in post - almost all hockey rinks have lots of white to use as a reference, and as Grabz mentioned a grey card will do the job. If that's your intended use, I would not bother; it really doesn't work well at all in dim light.
I really like my ExpoDisc. It's one of the best calibrated white balance tools out there. Sure, there are alternatives. Sure, it does no magic but it save me a lot of time in post processing of RAW. Also, shooting in JPG does not tolerate white balance mistakes Here is a review with some before/after images: ExpoDisc before/after
