evaluation service

I posted this once, but I can't find it so here it is again. I found somebody on Ebay that has evaluation service Tivo HDs. I am trying to find out what the advantages or disadvantages are. He said that they are already ready to go, that you dont have to activate them or anything. He also said that if you tried to do that that there is a possiblity that they might cancel the evaluation service on it. I just am not sure what to do on this. Hopefully somebody out there can help.

Thanks in advance.