"Comprehensive Immigration Reform" ("CIR") is a euphemism for an illegal alien amnesty, combined with supposed increased border security and supposed increased workplace enforcement. Some versions include some form of guest worker plan, an increase in bureaucracy to deal with the flood of applications, new ID requirements, and so on.
However, whatever the details, CIR is a scam built on a series of lies. Due to a sympathetic, corrupt media, CIR supporters are constantly allowed to jabber on in support of CIR but are never confronted with the hugely obvious downsides:
1. CIR would give a tremendous amount of additional political power to the Mexican government, far-left organizations like the American Civil Liberties Union, racial power groups like the National Council of La Raza, League of United Latin American Citizens, and MALDEF, and so on. Those groups currently fight against border security and workplace enforcement, and they aren't going to completely do an about-face if CIR passes. They'll continue fighting against the border security and workplace enforcement provisions of CIR, no matter what they say now.
2. CIR supporters say that the system does not work, which is highly misleading. The problem isn't our current laws, the problem is that most politicians are too corrupt to enforce those laws.
3. CIR would not be the last amnesty, and in fact would lead to more amnesties down the road. It's a way for the Democratic Party and various power brokers to obtain more power. They aren't going to pass up an opportunity to add to their power.
4. It will lead to additional illegal immigration. Whatever euphemism supporters want to use, millions - perhaps billions - of people around the world will see it for what it is: amnesty. Millions of those will try to come here, knowing that if they can stay under the radar long enough they'll be granted their own amnesty.
See also the DREAM Act (a mini-amnesty) and this discussion of the talking points used by amnesty supporters.