My company's 401(k) has the Brokerage Link option with Fidelity, and I just activated it this month. Our regular selection of funds only contained one Spartan fund. It was the S&P 500 index fund with a .07% expense ratio. A very good choice, but I wanted to diversify a bit more.
As far as I can tell, there are no specific fees involved in opening the Brokerage Link. They will assign you a new account number and transfer over the money you request. Once there, what you can buy may be limited by your particular 401(k) plan's options. In my case, I just wanted to add some money to a Spartan international index and a Spartan total market (US) index. I simply bought the mutual funds, since this is in a tax-deferred account and I won't be trading frequently or be concerned about distrbutions. There's a Spartan REIT index fund, if that meets your needs. I imagine it would be available in any 401(k) plan - but it's possible more esoteric options might not be.
Note that to get the lower expense ratio for the Spartan funds, you need at least $10k. You then buy into the "Advantage" fund, which is a bit lower fee than the standard "Investor" funds. Not a big difference, maybe .03%, but might as well take all you can get.
Edit, because I didn't write that very clearly. Like any brokerage account, you will have transaction fees when you buy things. It's just that if you are buying Fidelity mutual funds, the fee is $0. Because that's what they want you to buy, of course. Since that worked for me - no fees. Your mileage may vary. :)
