Hmm... I have somewhat mixed feelings about this. On one hand, I love the Gecko engine, given the large number of websites that are compatible with it. On the other hand, I find that Firefox uses one entire core of CPU, while the other cores almost completely idle, which makes web pages slow to load, especially if you are trying to load multiple pages at once.
We'll see how well this pans out. Hopefully, the newer Quantum engine will allow Firefox to run faster, use less RAM/CPU, will retain compatibility various websites, and will still work will most (if not all) addons. However, I realize there's no guarantee that any of the above will happen.