I'm personally OK with 'illegal immigrant,' but when you say a person is illegal (qua 'illegals') then that comes across as an implicit denial of legal personhood. This might seem pedantic, but the US constitution already distinguishes between rights of citizens and of persons in general, while many people who object to illegal immigration would like to deny the rights to accrue to all persons to those who have immigrated illegally, including myself. Thus, the term has become somewhat loaded to a greater degree than the authors of the current law may have intended.
As for terminology, IANAL. From my viewpoint if you illegally immigrate, through action or inaction, then you are an illegal immigrant. I've never much liked the term alien as it has also been used to describe little green men and such.
I think taking issue with calling a person illegal is splitting hairs, but I'll go along. The person isn't illegal, but their residency status is illegal.
And my last point, to reiterate on what I said earlier, I have no ill will towards illegal immigrants and know they have basic rights. I would expect they get treated fairly under our laws (well, as fair as out laws can be). That being said, they are not here legally and I would expect that changes the legal avenues available to them.