The HOV lane setup in Ontario is only good for long-distance traffic. In other areas there are dedicated HOV-only exits and entrances to local roads and larger highways. In Ontario you have to exit the lane and merge all the way right to exit.
For someone like me, the HOVs on the 403/QEW stretch is quite nice because whenever I use this stretch I use the HOV lane for it's entire length. It may not be the fastest if there's a slow driver in front of you, but it flows better and congests less frequently than the general lanes. Those lanes could be flowing faster at one point and backed up another, whereas the HOV flows at the same speed so it's more efficient.
I do think it's somewhat stupid to put in 1 HOV when they had room for 2 general lanes however. New builds with HOVs are fine, but when you have limited space you may want to squeeze as much lane space in instead. Conversely, if they just dropped the buffer (which isn't really needed, most other areas don't have this) they could have still 10-laned the 403/QEW, but with 1 HOV lane + 1 general lane.
On the 401, the HOV lane is an express lane within the express lanes. Seems kind of odd since you only have 2 express lanes now for general traffic and I could see that getting clogged with trucks with no way to pass them.