I bought a CDCO tools DRO 5+ years ago and it ran fine for 3+ years, then I sold that mill and I assume it's still fine. I bought an ebay DRO for almost nothing ($150?) to abuse on a project at work and it's functioned perfectly.
I recently bought a Chinese DRO for my lathe from "Reeson Tools". it was $400 including shipping for a DRO for my 13x30 lathe, with 1um resolution on Z and 0.5um on X and their higher end display unit (because it supports multiple tool offsets). I emailed them to figure out the order and they were responsive. I'm still installing it on the lathe but so far everything looks and behaves fine. My only complaint so far is the display only supports 16 different tool offsets. They also had 0.1um scales, but my lathe can't do anything with that (even 0.5um is pushing it). But for a grinder I would totally purchase the 0.1um unit, why not?
With DROs so cheap, it almost doesn't matter how long they last, as long as you can purchase drop-in replacements. Also I've priced 0.5um scales from Newall etc, and the individual scales cost more than $800, I could've bought two DROs for that price.
Unless you want special features or resolution, the ebay DROs are great. If you want something fancier, I found Reeson very responsive to my specific requests. If you're making a daily living on your manual machines, you can probably justify purchasing something name brand from a reputable dealer (Mitutoyo lists DROs down to 0.1um in their catalog, but their display units are extremely basic).