The express buses are basically non-existent on weekends. Sure, when the 25 is eventually extended to Kipling (or the 25F is made a permanent feature of the line), then it'll be practical, but right now, a 3 hr commute to downtown is awful on all fronts, and that's assuming you make the connection with the 21.
Personally, I think that the 25F should become its own route, treat it as a HWY 407 bus and extend it throughout North York. I can see usage there.
The questions then become how far will that weekend service go, and what time will they run at. Reducing the number of school trips might be ideal for Metrolinx, even though buses are fairly cheap to operate along the corridor.