I just bought one of the grinders and it was great for what I needed it for. They will work fine, just go ahead and get the extra warranty to go in and another unit when the first one fails...
One of the secrets of the Harbor Freight warranty is that you tell them it doesn't work and they give you another, no questions asked. When you get the new one, you can buy the extended warranty on THAT ONE and do the same thing again. I've been told by employees that they regularly get tools brought in a couple days before a date engraved on the side with the warranty expiration.
Their 20V Bauer line seems to work fine, and it's what I keep in my truck box.
