I remember this museum as being horribly dark and boring as a child. (My mom dragged me there on a regular basis!)
With the addition (done perhaps 10 years ago), it's really quite beautiful! The melding of the old and the new has created some great spaces which are well used.
We only had an hour as we waited on someone else in our party who had an appointment at U of T - and this seemed like the logical solution. As it happens, on Friday evenings after 4:30, they have reduced admission - bonus! So for $9 each we were able to go in, see what's changed and visit a few exhibits. Since I'm a huge bat fan, naturally the bat cave was our main objective - but we also managed to take in an exhibit on the dinosaurs, birds, mammals and life in crisis. That pretty much used up the hour - which we very much enjoyed!
Would I return: Yes
Would I recommend: Yes - the reduced admission on Friday evenings is a great start to the weekend.