If you are talking about carbon clinchers, I would recommend you search Velonews for their discussion about a month or two ago about this topic (I believe it was in Leonard Zinn's column). The problems with braking and delamination with the heat from braking were addressed by someone from Zipp, and it would make me very leery of wheels with uncertain provenance
You are probably OK with tubulars, but even then I would be reluctant to go there. One good high speed crash will use up your savings in copays in short order.