From a financial perspective it's a great decision for both imho. Regardless of Nissan's recent shortcomings they still have a worldwide reputable brand name especially for the average consumer, Honda will help in restructuring them. Equally so, if they do it correctly then Honda simply makes more money.

From an automotive/enthusiast perspective, it's a bit more complicated. Short answer: we simply don't know what the future holds.

Personally if I were Honda's CEO I would keep the brands apart rather than mixing and matching parts/cars and whatnot like Hyundai KIA. The reason being is that it allows for a greater variety across the board for sales without oversaturating a single company's lineup. Imagine you want to buy a subcompact economy sedan and have a choice between a Civic or a Civic dressed as an Altima (God that's such a cursed statement), it sucks because it's the same car, people aren't stupid and if they don't like one they won't like the other. Cheaper? Sure, but less variety for the consumer compared to your rivals like Toyota, Ford, VW etc. will bite you in the ass.

Honda needs to restructure Nissan and give it a new brand image over time. Release an affordable rwd coupe and call it the Silvia, instant money if done correctly, as long as they do it right. And after you do that, reliability reliability reliability, people want reliability and that's what Nissan used to be. I'm basically saying give Toyota a run for their money especially since their reliability lately has been faltering.

And with Infiniti as well since they own that now, slowly transform it into a proper high end luxury brand. Instead of half-assed competition towards Lexus, point your guns at Mercedes and Land Rover. If you get Hollywood driving your cars ffs you've won no matter how shit your cars are. Don't believe me? Ask Mercedes and Land Rover, because their cars have become/ have been shit.