The Cold Steel poly bokken does not have a good feel...I trained w/one for about an hour and found it very inferior to Japanese Kashi.
We use the PolyPro bokken for kumi-iai work with plastic sayas. It has less mass than a bokken in case someone gets his fingers or hand smacked accidentally. Other than that it is not very useful. It has so much flex that if you swing full power and then stop your cut abruptly at the same distance you do with your wooden bokken, it can actually flex enough that you smack your partner. Very specialized use in my opinion.