Just bought a half dozen Seimitsu sticks for some cocktail cabinets I'm building. Never tried them before this point, sticking mainly with Happ/IL. They where like 15 bucks each so I gave them a try.
They are definitely smaller than Happ/IL sticks, albeit a tiny bit wider on the mounting plate in the width direction, it's shorter in the other. you can rotate the stick to use this to your advantage if you'd like to get some buttons in close or whatnot.
for some reason the switches with the Seimitsu don't have a normally closed tab connection, only a normally open. So if you have a wiring scheme in mind that requires this, (LED lights or multifunction switching or something) it's something to be aware of and you'll need to swap them out with some others.
the plastic ball tops seem like they could be a tiny bit better quality (small pits where the injection moulding takes place and sometimes a small amount of seam flash) but they look okay. Both can be buffed out if that's your thing. The threading on one of the balls was a tiny bit galled and I had a tough time threading it on. I ran a tap through it and didn't have an issue with it.
otherwise, Seimitsu feels like a solid stick even with it's cheapness. Thumbs up from me.