Short version:
If your company provides you with an excellent plan at a low cost to you, and if you expect them to continue to pay for it despite huge increases in cost to them, then selfishly(not necesarily a bad thing) you might want to be against any form of universal health care. About the only drawback is that if more and more people don't have healthcare we might see a whole lot more TB and other communicable diseases which you could catch.
If your company is struggling to provide you with health care, reducing converage, and shifting costs to you, then you should be FOR the public option. Private health care companies are using extraordinarily large amounts of health care premiums for "overhead" like salaries, adverstising,etc. I mean huge amounts, some over 30 percent of your premium. Medicare only needs 2 percent for overhead. The idea is the public option will force down the overhead of the private companies. So you will win, your company will win, your health insurance companies stockholders lose.
If you privately pay for health insurance, then you should be for the public option, for the reasons above.