I think he is talking about something like that also. http://www.co.cc/. The pros of having a free subdomain it is free and you don't have to pay a yearly registration fee. The negative is that you are using someone else's domain and will look less professional if you are using your site for a business and or an organization.
And that is the problem with ANY subdomain, whether you are paying for it or not. You can actually BUY subdomains under domains like uk.com. The company that sells uk.com subdomains (Centralnic) has been around for a long time and is generally believed to be solid and reputable... but the fact remains that their domains are subdomains and not realy domains. And I would certainly steer clear of any company that is giving subdomains away for free... you have no rights whatsoever if you obtained something for free.
Centralnic domains may be worked out as they do not give it for free. They even accredit registrars for the same. So there should not be any hassle in going for them.
I have no problems with Centralnic as a company - solid, long-established, reputable.
I don't agree with their "ploy" of selling subdomains and fooling people into thinking these are real domains.
I am a member of a national organisation in the UK (nothing to do with domain names) which recently changed its name from one three-letter acronym to a different three-letter acronym. It used to be [oldname].org.uk, it is now [newname].uk.com. Makes me cringe!
if you plan just a blog ,it is ok, even blogspot and wordpress offer subdomains, otherwise it is not very advisable, as you will not have complete control, unless it is a second level cctld or something similar,
.co.cc also gives free subdomains