I used a 24' extension ladder once, just to see if it made the job go faster. When I was done I thought "If I hadn't used this ladder I would have been done 30 minutes sooner, and I wouldn't have to load the ladder back up".
The only way I would use one now is if I was trying to gain access to a limb over a house where I could use the ladder to get on the roof. Other than that I think it's just as fast or faster to ascend a rope, since you already have the rope set, and by the time you set up the ladder you could already be in the tree if you climbed the rope.