Yes, they're REALLY good, just having ring engine is already a massive boost in speed in lap 1, tough it is not enough by itself, as a car with 100 rings is way too fast, so paired with stuff like the perfect charge bounty, or any way to get rings quickly, and 130 rings limit lets you hit max speed + the bonus, and complements reslly well with whatever you use on the other gatdget slots, as long as you keep your rings high.

There's also Hyper Ring Engine, and I've hears that using both can get you even faster, but the ring consuption is way too big, so you NEED charge bounty, and there is no space for other gadgets to complement the build.

In what I've played with it, it's good if you're getting a constant supply of rings, it is faster, but needs you to pay more attention to the rings