If your employer matches your contribution then absolutely! (That is, if you contribute 4% of your pay to the 401K, then the employer will contribute another 4%.) That's the easiest money you'll ever make. Contribute at least up to the maximum that your employer matches.

Now while the money you contribute is yours forever, there could be fine print on the company match. For example, if the company contribution only "vests" (meaning it becomes permanently yours) after 3 years, and you don't expect to keep working there for 3 years, then it might not be such mandatory, but it's still a good way to invest tax-free.

Anyway the usual rule is to contribute up to the match if at all possible, to get more of your employer's free money.