I didn't stick around long enough to qualify for retiree health insurance. But I think the main advantage (besides cost savings) is since it's group, through w*rk, they accept everyone regardless of pre-existing conditions during their open enrollment. The disadvantage is since it's through w*rk as benefit, the megacorp could decide it's not in their best interests to continue offering the benefit. Where as, if you obtain insurance in the individual market, that's a contract between you and the insurance company (such as Blue Cross).
Fortuantely for me, my health is good enough to qualify for individual health insurance when I decideed to FIRE. Otherwise, I'd still be stressing at the cubicle at w*rk, waiting til Nov of 2011 when I would have qualified for retiree health benefits. That is, if I wasn't let go beforehand. But that's a whole different disscusion.