You cannot register .mx, only .com.mx, .net.mx and .org.mx. Mexico's TLD is controlled by a private university, Instituto Tecnologico de Monterrey (ITESM). They created a 'separate' entity to administer them, but they have little flexibility in their pricing and reseller structure. A few .mx domains were issued to public and private universities only (
www.iteso.mx,
www.udg.mx,
www.uag.mx). The .com.mx extension has no restrictions. You have to be a legal registered ISP or telecommunications company with the Federal Telecommunications Comission (COFETEL) in order to register a .net.mx. Somewhat similar rules apply to .org.mx (you must have an official organisation record).
There are, however, some advantages to nic.mx's domain reg. system...
BTW, I don't find .com.mx to be a very attractive TLD, as they have only local scope (Mexico). Most companies try to avoid .com.mx if they can get a hold of the .com, because it's a rather restricting extension. People in Mexico still will try the .com first, followed by the .net before the .com.mx when looking for a company's website. Also, domains are overpriced ($35.00 USD a year) for what I consider a 'subdomain'.