Not only can it help you venture into the Dungeon multiple times, but all your other creatures also get +1/+1 when you finish a Dungeon
That said, there are a number of factors that might make you consider small adjustments
Just remember that Rex's ability is compulsory, so make sure it isn't alone on the field with any 0-cost cards or you'll lose them
Grindstone mills two and repeats whenever two milled cards share a color so with Painter out, it never stops and decks the target in one activation
Unlike conditional counters, it never sits dead in your hand waiting for the right target
Based on what Ive seen so far, you could do worse than a mono-ruby deck all about Sid and the toys he tortures for fun, but you can also dip into emerald or amber (preferably amber) to get access to a wider array of toy tools to play with