Okay, first things first-- I LOVE these things. I got the 10" wheel from their kickstarter and I was a little dubious about whether it would live up to the hype-- but it DID. I wanted a little more intensity though, so I ordered up the 6" wheel as well, and I loved it TOO. But-- supposedly the Chirp Wheel is rated for a load capacity of up to 500 lbs. Well, I'm 5'8" and weigh just shy of 190. I don't do anything exotic with the wheels and I use them as recommended, on a firm surface (wood floor).
The 6" wheel cracked across its width after I'd been using it daily for a month or so. I have got NO complaints about Chirp's customer service! It's been fine! I sent them an email and a photo of the crack in the wheel, and they sent me another one. This one lasted for a couple of weeks or so, then IT cracked too! Same deal-- email, photograph, they sent me another one. This one has been going strong for a few months now, but last night, it cracked too. Just a little hairline crack and I decided I'm just going to superglue it, put a little gorilla tape over it and keep on using it. If it gets worse, I'm just going to buy another.
So the final conclusion-- Love these things, they are good for what ails my back and I don't want to live without them. Chirps customer service has been great. I just hope that they are making the 6" wheels with thicker plastic or doing whatever they need to do to make them stronger and more durable. Because I'd like to not have to keep replacing it. They are definitely not up to a 500 lb capacity. On the other hand, the 10" wheel has been fine and I'm still using the very first one on a daily basis with no problems. I've recommended these things to several of my friends who complain of back issues.