I've sat in on a Guideline demo. It is amazing.
I'm on my company's benefits committee. Survey last summer showed that employees wanted a lower cost 401k, and Vanguard was specifically mentioned several times.
I wrote up a data-driven paper to management. They agreed to explore other options.
We just completed the proposals from 401k providers, Guideline was in the mix. We also looked at Betterment for Business, Employee Fiduciary, and traditional brokers like ADP, and the incumbents gave us "better numbers" which are terrible compared to Guideline.
This week our VP is presenting her findings to management. Fingers crossed we get the thumbs-up to proceed to switch to Guideline.
Update: we transitioned to Guideline from 1 May 2019. Fuck you American Funds and Wells Fargo "Advisors".