What do you guys think of fxaix?

It's an index fund, so as the S&P 500 goes, so does it. I owned it for a year, from early 2018 to early 2019. Along the way, it paid dividends quarterly at about 2%. Not a bad fund, highly rated on Morningstar, low expenses because it's just an index fund and it is Fidelity. The only reason I sold is that I found my portfolio was too closely tied to the S&P 500, so I moved away from the index funds and picked up some bond funds so I could withstand if the market plunges like it did in the last quarter of 2018.

What was your initial buy in? Im.nkt using my 401k in it

I was actually in another Fidelity that had a minimum 15K, then after a couple of years Fidelity converted into FXAIX last year. It was basically the same fund with a lower fee.