I have three of the Ryobi's and a couple dewalts and a makita variable speed. If I had to have just one, it would be the Makita. The makita is the best grinder I have ever used.
One of my dewalts started on fire...it was the cheapest version they sell. My other dewalt is great and after about 10 years I replaced the brushes cause they were done and it seems good as new. I would also buy one of these again, it has been good but not as nice as the makita.
Now, for the ryobi, HD had some crazy sale one day. 3 boxes taped together and sold for the price of one. Not sure why but I picked them up. I keep a flapper disk on one, a diamond cup wheel on one and a thick grinder disk on one. They seem to work okay but are louder and not as smooth feeling as the dewalt and makita. I had a fourth but it burned up.
My makita, dewalt, and the ones that died were all used heavily in cutting ceramic tile, stone, and concrete so they lived very abused lives. I would expect with normal use they all would have lived longer lives.
I have used the HF grinder and was not impressed with it. The ryobi is a little better in my opinion. The dewalt and makita are a lot better than the ryobi. I would not buy the warranty unless you are a heavy grinder user and abuser.