iv built my whole suit out of yoga matts mostly because im in the uk and thats all i can get. you have to be careful because there not as thick as what they guys here use so your suit can warp if its not looked after properally. also if you cant afford plastidip which is expensive in the uk you have to seal with pva glue which in the yoga matts will be prone to cracking under to much stress. but its doable
What firebapx said - yeah I've got an IM and a War Machine from yoga / camping mats on the go for my kids - not the smoothest finish although the PVA helps - that said depending on the mat you buy, some have a much better finish