@Scott Trench,
You really created a lively discussion with this question.
I am a strong proponent of using the gift of tax deferral to grow retirement wealth. Because the contributions you make a 401(k) are pre-tax (or potentially Roth), and because the investment gains are generally not taxed until you take distributions in retirement, the amount of wealth you can acquire is significantly boosted. Think of the tax sheltering in the same way you would leverage on a real estate deal - it supercharges your cash-on-cash returns.
The comments about employer matches are spot-on if you work for an employer. Free money is good, period. If you are the employer, and have no employees, a Solo 401(k) is a wonderful tool as well.
With a self directed Solo 401(k), you can have the best of both worlds - the ability to set aside income into the plan on a tax deferred or Roth basis as well as the freedom to invest in real estate and achieve the types of superior results we all like to brag about here on the BP forums.
Many of my clients have a 401(k) through their employer as well as a Solo 401(k) sponsored by their personal real estate business. They contribute just enough in the employer plan to max out the employer match, and then defer some of the income they receive from flipping houses personally into their own Solo 401(k), thus reducing their tax exposure in that enterprise. The employee contribution limit of $17,000 for investors under age 50 or $23,000 is split across the two plans, but the Solo 401(k) can accept profit sharing contributions independently of what may be occurring in the other plan. The icing on the cake is when they take the tax-sheltered funds in the Solo 401(k) and put those into passive real estate investments such as rentals or notes.
I must, however, throw a bit of cold water on comments touting the benefits of taking a loan from your 401(k) or other qualified retirement plan in order to invest in real estate. The factor most folks miss is that you are giving up on the initial savings achieved when you made pre-tax contributions to your 401(k). Sure, you can borrow from the plan without taxes or penalties at a low interest rate, but you are repaying the loan with after-tax dollars. If you are in the 28% tax bracket and borrow $50,000 from your plan, it effectively takes $69,000 of new earnings to repay the note - not including the interest. Most hard-money lenders charge a lot less.