Sorry for "glass"-definition. Split case is definitely more appropriate, but I do think that it must be made clear 6MT split vs. STI as those two are _totally_ different transmissions.
I had 5MT before and some people called it "glass-box/transmission". I thought that it was exaggeration as there were lot of 5MTs with decent amount of power.
Honestly 5MT is not a bad box, but it's much more likely to break comparing to STI-6MT. My 5MT lost bolt in front diff which created a hole in casing. I don't know for sure the history of the car, but it was never "tuned" so we're talking standard power, but possibly abusing driving.
Why I'm so adamant to highlight the difference to STI-box is that the 6MT from STI actually IS really one of the really strong transmissions out there regardless of the brand of the vehicle.
By FAR the biggest improvement in my car has been STI-6MT with diff locks (Suretrac+visco+Suretrac). On asphalt this is not that much of a difference at all, but now winter time it's difficult to overemphasize the difference. Sugar coating for all this is the feel of the shifter in STI 6MT - you just WANT to change the gears as it's so addictive and transmission also feels like it will hold.
Coming into corner on snow or ice (or gravel) - flick the clutch and power slide through. Just simply beautiful and an absolute joy to drive
5MT with centre+rear visco was nothing like this at all.