I have the Mavic Ksyrium Elite 650c on my tri bike. Mine go out of true more often than they should. On 40 plus mile rides they get uncomfortable. They transfer more road noise up to my hands/elbows. I like them for sprint tris though.

I can't comment on the Isis, but I have no problems getting tubes and tires for the 650c wheels on the internet. My LBS has tubes in stock, and I live in a small city.