The generic office store glass desks are crap.
I like glass surfaces, but you definitely want at least 1/2" thick glass. Also be aware that something like that weighs a ton. My coffee table is 1/2" glass, and it's about 40" round and the top alone must way.. hmm.. 50-60 lbs?
The problem with glass desks is that they don't hide ANYTHING. So all your wires, etc, etc will be all over the place. Also, while they're very easy to clean (windex, duh) they are difficult to avoid getting dirty. For instance, I place glasses etc all over my regular desk and clean it maybe once a month. On my glass tables, every time I set a glass down is another condesation-residue-ring that I'll have to clean up later. So I guess overall.... not sure if it's the best idea. But they can look very nice.