A quick google search found a thread where someone was questioning whether apple snuck it in without touting it as a feature. They were going off pictures and personal use, so I think its safe to say the glass doesn't give any huge improvements.
People have suggested that the MBA's thinness preclude the hardware below the glass trackpad. I don't know if it is true. I do wish mine had one though. The button is does have is quite thin/narrow and a *bit* awkward to use IMHO. Tapping on the pad works, but mmmm the feel of a good click.
Had the use of a unibody MB last year for a few months and hated that 'glass' trackpad with a passion. The tension/travel across the pad was so uneven you'd swear it sloped, all of which meant right-clicking and gestures were very much hit and miss. In general it just felt unnatural, and the loudness of the clicking (clackety-clack) surprised me....not in a good way either. Didn't even feel any different, maybe I was unrealistically expecting a nice smooth glassy look or texture, but considering the amount of fuss they made over this 'feature', I was far from impressed. Really became an irritant.
Now I'm onto an MBA and the trackpad feels identical. With one exception. It actually works infinitely better than the MB glassy one. Natural, multi-touch gestures mostly, or if I want to use the unobstrusive button with my thumbs I can easily. Perfectamento.
So my moral for this thread is, be careful what you wish for...