We've done some. Primarily because the ballast was out in the fixtures. I would not do any that weren't direct wire. I ordered some from 1000bulbs that were 3000 lumens at 5k. Beware that many retrofit bulbs have a lower lumen rating than the good Sylvania bulbs you were probably using before.

Someone mentioned it above, and a friend and I were talking about it last week that we don't think they'll last as long as a dedicated LED fixture. If you feel them, they still get pretty warm on the ends. When we re-lit our shop 3 years ago, I found much brighter LEDs for just over $100. Retrofit was going to cost around $60 plus much more time, and we were adding 1/3 again the lights we already had so would end up with different fixtures anyhow. I believe those Lithonia lights have gone up 50% since then though. I think the fully sealed LED fixtures will remain brighter over time as you're not relying on a reflector like the T8 fixtures.....at least the ones we had. It was surprising how much brighter those were if a guy cleaned the reflector.