Extremely low fees on a variety of funds, compared to other services out there. Plus it's a long-standing, trustworthy name.
The website feels old-fashioned and clunky. The netbenefits side for my employee 401k is beautiful, but switching over to look at other accounts' information feels like I'm stepping back a good 10 years in website design. It may seem like a petty thing to be concerned about, but when the customer-facing tech isn't great, it does raise some concerns about how up-to-date (and secure!) the back-end is.