From my experience, it's good at showing what seats have been selected and are available - if a seat shows as available, then I've been able to select it as soon as I make my purchase.
In other words, it seems to be driven by the same data as the Seat Selection tool itself.
What a lot of people talk about with it, is that's it's not accurate for identifying how many seats are available, as there may be many people with unassigned seats.
So for seeing what specific seats are available, it's good - for fare availability, it's EF.