401K is all about two things for me: 1) Matching and 2) Planning
Matching: As soon as it was available to me, I signed up and contributed enough to get the full matching amount available from my employer. After 6 years I was fully vested which means that my money was doubled in terms of contributions and all funds became mine. Now, if I wasn't gonna keep a job long enough to become fully vested, I would take another look and plan accordingly.
Planning: 401K's are just like any other tool in the box: They have a purpose and can be used to great advantage but if they're used incorrectly they can make the situation worse. You have to consider employer match, veting schedule, years to retirement, tax deferment benefits, etc. The younger you are and the more time you expect to be able to stay in the plan, the more options you have to make good use of 401K's but with good planning and understanding of the options most folks can find benefits in a decent plan.
Of course, if you have debt and expenses RIGHT NOW that will drive up your monthly expesnses or create more debt if you put money into the 401K, you have to consider if the 401K will provide you with greater income than the debt liabilities that will be created by not having that monthly contribution for current expenditures.
Like most thing, it's all grey, a balancing act and every situation is different and every person has different goals and risk/security thresholds. If you're really uncertain, get educated: If you have a 401K, you should have someone who is the plan administrator - talk to them and enlist their help in understanding your options and maybe even talk to a tax and/or financial planning professional. You want to make the best of the tools available to you and to do that you need to be informed and understand your options in the context of what YOU want to do.
Oh, and in addition to my 401K, I also try to keep at least 3 months take home pay in savings...because you never know.
One more thing: Get debt free before investing in anything. Nothing makes less sense than having money in savings/investments that gain you 3% year when you have debt that costs you 10%/year...that's a 7% year loss on your investment. Get debt free, THEN save/invest.
Best of luck!