We'll get a better idea of ridership levels in the near future, but how many people who take the extension can safely say that the ridership of:
VMC is over 14K PPD
HWY 407 is over 3K PPD
PV is over 17K PPD
YU is over 30K PPD
FW is over 17K PPD
DVP is over 2K PPD?
If any say yes to any of these numbers, then I'll say this line is a huge success, because it means that ridership is still growing, despite being fairly strong initially, despite GO's continuous improvements to the Barrie line, and despite fare integration all along the line. While some may criticize HWY 407 and DVP for having really bad ridership, they are present for a future use. When I think of Downsview Park and it's comparison to the GO station ridership, 2K PPD is really good for most go stations, but terrible for TTC stations. Maybe they overbuilt the station for what it was planning on seeing, but we won't know until RER is up and running, if ever.
These numbers also point towards the huge need for fare integration, it shows that there is an extreme potential for this line to be really busy, since commuters are already choosing the subway over GO or driving despite the insanely high YRT/TTC fares, and the need for GO passengers to use the TTC now.