I think it's wrong to equate an illegal immigrant as an "invade." There is a difference between someone who wants to pillage and rape your villages and someone who wants to do crappy jobs that no one else wants to do in order to fulfill their own "American dreams" of a better life. It's not at the expense of everyone else. Sure, they "take our jobs," by perhaps working at Walmart, but they also create them since they buy TVs and clothes just like everyone else.
I think skilled immigration should be unlimited and advocate greatly increasing unskilled as well. It has to be somewhat gradual, otherwise it'll put strains on governmental services and the economy.
As I said, immigration should be easier. I also don't think they 'take our jobs', they instead probably generate jobs, and they take the ones people born here don't want. I think having people come here is good overall, as it provides cheap labor that helps reduce the trade deficit. I do think we shouldn't hand out lots of government benefits to them - they should live just like everyone else. If they want to work, let them in to work, but they better work, otherwise they shouldn't expect help.
However, as I said, I don't like it when people disregard our laws. Certainly, they are not the worst of invaders, but they are invaders - they enter our land without permission. If you wish to live somewhere, you should respect the laws that have been put forth.