Without getting overly excited; I believe Finch LRT is a useful line to build.
1. The right-of-way will speed up the travel during the peak periods, when the street is congested. And, will hopefully make the headways more even.
2. The capacity limit is higher than what can be achieved with buses (even if buses are given dedicated lanes, like YRT's VIVA); the fixed wheel advantage enables longer vehicles.
3. The cost is high, but that reflects the general inflation of construction costs. Surface light rail is still cheaper than subways. Once we get the final per-km costs of the OL, SSE, Yonge North subways, they will end up being 3-5 times greater than the per-km cost of Finch LRT.
However, I don't really like it when Finch LRT is marketed as "rapid transit"; that's misleading, and could lead to negative public sentiments once the actual travel times are known from the line's operation. I would stick to "improved, high-capacity local transit" instead of "rapid".
Or, if anyone insists on calling Finch LRT "rapid transit", then we should call TTC's express buses "rapid transit" as well, since they operate at a similar speed.