1. It's one of the very best ways to save for your retirement.   Few alternatives are better.
2. There's a good chance that if you participate in one your employer will just give you money for doing so.
3. You should start contributing at as early an age as you can.
4. You probably want to put all of the money that you defer into an equity index fund.
5. I like moving my money between sector funds.   When the sector is low, I move my money to that sector.   When the sector recovers, I move my money to a different sector.   Most 401K plans allow you to move your money several times a year, at no cost.

You should only borrow from a 401(k) plan only as a last resort.   Why?   You may change jobs before the loan is repaid.   If you do, that loan is immediately payable with interest.   If you fail to come up with the necessary cash for that repayment, it will cost you.   Moreover, it is classified as a "deemed distribution" from the 401(k), and you will be taxed.   To make things worse you will be penalized for an early withdrawal -- 10%.   Do I have any more "good" news?   Sure, the interest you pay yourself on the loan comes from money on which we've already paid taxes.   But for 401(k) purposes, it will count as untaxed earnings.   That means we will pay taxes on that money AGAIN in retirement when we make withdrawals from the 401(k).   Thus, while the loan feature is attractive to some, it's really something to use only when all else fails.

IBM's 401K

Soon, I will have a matirx of the available funds with buy/sell indicators.   Using this matrix as a guideline, you could perform your own technical analysis on the funds in your 401K.   By moving between the various funds, you will be able to maximize your anual return.