Illegal immigration is not a big issue for me because I have a middle position. The extreme positions on both sides are simplistic, and those who adopt them tend to ignore any evidence that undercuts their often fanatically held beliefs on the subject.
Last night while I was driving around working, I caught bits and pieces of an NPR discussion/debate on illegal immigration. I didn't hear the introductions, but on the anti-illegal side was some state senator (I think), with an ACLU representative taking the pro-illegal position. The senator portrayed illegals as basically an evil horde of invaders who depress American wages, suck up social services, and commit crimes. The ACLU guy refused to even use the term "illegal alien" -- since of course we can't offend people who commit illegal acts by labeling them illegal -- but referred to them repeatedly with the ridiculous euphemism "undocumented workers." To him, illegals are all great people who help the U.S. economy. Both guests referenced only cherry-picked data that supported their own preconceived notions. The ACLU individual, as is typical on the left, basically accused those who disagree with his position of racism. The senator overemphasized a few sensational crimes committed by illegals as evidence that they represent a lethal criminal threat to American citizens.
In reality, illegal immigration has both benefits and costs, neither of which is easy to quantify. It is obvious that most illegal immigrants come to the U.S. to find work, and make up part of the workforce. By working, by purchasing goods and services, and by paying indirect taxes, they clearly contribute to the economy. At the same time, however, they impose costs, particularly in terms of social services like medical care. The actual cost-benefit ration probably depends on each region in question, possibly right down to each individual locality. As for crime, although most illegals come to work, as with any population, a small minority are prone to criminal behavior (not counting the crime of illegal immigration itself). For every wave of immigrants that enter the U.S., some will be criminals. But this applies to both legal and illegal immigrants, which undercuts the crime argument as a factor in keeping out illegals -- unless one opposes all immigration. Again though, it depends on locality. If an area has a particularly high number of criminal illegal aliens, obviously that area is going to view illegal immigration as a major criminal import problem.
Illegal immigration is both a problem and a benefit. Which side outweighs the other depends on your outlook, the statistics you choose to accept, and where you live. If you own an office building cleaning service in New York, staffed with hardworking illegal immigrants who do a good job for low wages, your perspective is probably quite different from a property owner in Arizona whose land is constantly trespassed on by illegals.