Is the United States considered a good place to work, now that Poland is in the EU?
No... most of young people prefer going to EU. It's much cheaper to get there and if sth go wrong you can easilly (at least in most cases) get back to Poland. You can make good money here in Europe, especially when comparing to USA while dollar is so cheap... Polish people still need visa if they want to go to USA, and if they want to work, some company from USA have to send them invitation.... there is so much paper work. All in all EU seems to be better choice for young people.
How many young Poles have seriously considered coming to the States, and staying, legally or illegally?
10 or 20 yrs ago - many, now.... I think that would be <10% of young people who want to leave Poland.
Do you know people that have come to America, and stayed to work, or start a new life?
Yes, part of my family... they moved ~15 yrs ago, they live there, some are doing just fine, some aren't....