In my view, .XXX will open the door to censorship.
In regards to adult sites, .XXX will prove troublesome - many of them will try to operate on both .com (or whatever TLDs they're using now) and .XXX - seems like a reasonable solution with no real risk.
However, some big problems ... some may be solveable?
1. Many adult sites will be unable to get the corresponding .XXX - a possible solution is a sunrise period for folks with corresponding domains in other TLDs to register a .XXX ... may happen, but I doubt it for both economic reasons and more so because how would conflicting corresponding registrations in various TLDs be handled ... ie. sex.com, sex.net, sex.org, etc ... which should get sex.xxx?
2. Adult sites may be required to actively operate in .XXX only by credit card processors, etc - here again many adult sites could simply setup their .com, etc TLD domains to point to a .XXX ... such a scheme would help existing sites transition over to .XXX
3. The biggie ... laws/regulations may be implemented requiring all adult oriented content to located on .XXX only - in that situation, the corresponding domains in other TLDs / redirectors would be of minimal use, since all adult oriented content would be required to be in .XXX only.
4. Lawsuits/political pressure (relates to
#3 above) basically forcing adult oriented sites to operate in .XXX only.
5. Assuming such sites are able to operate in .XXX with no problems, they may have a big problem getting customers since many internet security/monitoring programs (some likely based on the ISP side; more difficult to hide where one's been from their ISP) will likely allow one to easily monitor/block .XXX domains - such as by a spouse / girlfriend / boyfriend, parent (many adults live with parents) etc.
Even assuming .XXX works out perfectly, it's still likely many people will run into censorship as other restrictive TLDs are introduced, such as .KIDS, .ADULT, .FAMILY, etc - possibly leading to requiring all webpages to be content rated; self-censorship is often far more extreme than government mandated censorship.
Ron