Google Mail feels like it's transmitting over a 57.6k modem these days. If developers at Google can't make fast JavaScript front ends, what are the odds that an average front end developer can?
And yes, why wouldn't I? My 50Mb/s cable modem can download the fully featured Fastmail (500K) 13x faster than GMail. That's how bandwidth works.
It's kind of sad because it was the only webapp that made sense to me.