I started a 401k here with rollover money from my last employer. They'd used Vanguard for those investments. The rollover was a breeze and the service has been great. The broad selection and low cost of the funds really helps boast returns - or minimize losses :(.

MY company uses Vanguard, and overall I have been satisfied with it. However I had a very hard time initially accessing the website. I was locked out, then called and ended up being locked out from phone activation. It honestly took two months (of very sporadic attempts) by me to get online access.
Once that was over, though, I have had no issue. I didn't lose very much over the downturn, so that is a bonus.
I did receive something from them recently which puzzled me. They are offering a fee based account management service. The only thing is, as far as I can tell from reading the literature which they sent me is that they are going to use the same program which you have access to for free right on the website.

I have one 401K account with Vanguard. I have had it about 8 years. I have found their statements very easy to understand. The investments are hard erto transfer however.than my Principal account. Customer service is very helpful!

My previous employer, Cambridge Associates, used Vanguard for its 401k plan. Cambridge Associates is an investment consulting company that helps many of the country's most important non-profits (think big hospitals, museums, ivy league schools) and wealthiest families manage their money. It's Cambridge's job to pick good managers - and Cambridge picked Vanguard for its own retirement funds.

Why?

Most importantly: Vanguard offers a wide selection of low-cost funds.

Also: I found Vanguard's website and customer service to be superb.

Though I no longer keep my 401k with Vanguard - I do keep an IRA and Roth IRA there. I'm a fan. Investors who are a lot smarter than me are also fans.