I have been an at will employee at my last two jobs. The first I was with from early 2000 ontil early 2007. I left on good terms giving my 2 weeks notice, not because I had to, but as a professional courtesy. I know for a fact this employer has let people go without cause or notice, sometimes highly justified, other times not so much.
The place I am with now I have been at since late 2006 (there was a few months of overlap where I was working both jobs full time). This comapany is much larger and although they make it clear we are at will employees, I have never seen them terminate someone without cause or notice. They always have write ups or associated paperwork, and the option in most cases is given for the employee to tender their resignation.
Unions suck. Period. I have held union jobs and the union never promoted a productive workplace. It promoted laziness and an untouchable sense of security for the most worthless employees. We had one guy who transferred down from another state who was employed with this company for 5 years. He had a drug problem and was taking oxycontin at work in dosages that could put any normal person in a coma. He would either be an incoherent drooling mess or pass out at his terminal completely. The company could not drug test him, without written consent from the local union rep. Only after witnessing this behavior did the union rep allow the test, and even then the test had to be scheduled and could not be random. Of course he failed, even with the test being scheduled for him 30 days in advance. Rather than being terminated as he should have been, the company was forced to send him to rehab and let him relapse THREE times before termination of employment was allowed. This took a timespan of almost a year to complete where we were either shortstaffed because he was virtually unconcious at his workspace or away at rehab. In my eyes, this company should have canned him and sent him to rehab as a good gesture for him to get himself together right after an on the spot drug test.
All BS aside, if you work for a good employer and are a good employee, being at will should make no difference. I know I have never given it any real thought until this post. Just my .02.