I have worked with a bunch of Mexican immigrants, most of them illegal. I'd agree that they are very hard-working and mostly law abiding, and most pay taxes (but not all of them do; many don't, and many send a huge portion of their pay back to Mexico).
I also worked with a Mexican guy who was brought here as a minor. Worked his butt off, attended high school here, owned a house, paid taxes, no criminal record, great guy. He was far more "American" than "Mexican" because he had spent most of his life here. Sadly, he is classified the same way as someone who comes here illegally on their own accord, despite the fact that his parents brought him here as a minor.
Point is, this is a complicated used, and unfortunately congress sucks too bad to do anything about it.