One thing I really like about one in particular, the Corsair K90 is that there's no gap beneath the keys to get crumbs etc lost in the bottom of the keyboard.
Then there's the keys haha. More consistent. On normal boards you often find a key that wears out and isn't optimal anymore. Mech boards are supposed to take something like 50 million key strokes per key. It's hard to describe the difference to be honest.
Are they worth the price tag? Compared to 'gaming keyboards' which often cost a fair bit anyway, absolutely. The price hike isn't too much. Compared to lower price stuff like the Sidewinder from Microsoft.... well if you're looking at the sidewinder you probably can't afford one anyway.