I have the OEM side cases. On my previous bike (BMW F800GS) I had a set of aluminum Touratech Zega panniers. And I have mixed feelings about both.
The OEM Yamaha cases are tidy, relatively easy to use, and very functional. One thing that annoys me is that you need the key in the pannier to open and close it (whereas the Touratech ones had a lockable latch, but you could open and close an unlocked pannier without a key. )
The Touratechs were way burlier, but probably overkill for 95% of my riding. The Yamaha panniers are a little ersatz (plastic but with aluminum "paneling"), but as I've ridden and used them, they provide all the function I really want or need.
For a low side or tip over, I'd rather have the burly all metal boxes. But for the simple "schlep stuff" factor, the Yammi's serve just fine.
FWIW I've never been a top box guy...I don't like the optics and haven't needed that much storage. But I did get a Kriega tail bag which has a great mounting system, is totally waterproof, and gives extra carrying capacity. I really like it. Here's my laden S10 from a road trip last summer. Hope this helps.