I like some things about the R games. I do think they are pretty mediocre for the most part. I think they're worth a play but there are far more boring games in the series. The Land games tend to recycle the minigames and of course there's Act Zero.
Most of the games play the same so I don't think the R games are really much worse, just more of the same, bland gameplay. At least they attempted to add new things like Castle, Grand Prix, and Battle 64. It's something.