We have a Ninja Professional and for what we use it for, it is sufficient. It's definitely much, much, much more powerful than a typical blender and seems to be able to do most of what a VitaMix can. I haven't tried to use it for milling flours but I make nut butters and smoothies with no problems at all.
'Life should not be a journey to the grave with the intention of arriving safely in an attractive and well preserved body. But rather, to skid in sideways, chocolate in one hand, wine in the other, body thoroughly used up, totally worn out, screaming 'Woohoo! What a ride!' So every day is a holiday and every meal a feast."
What will you use the vitamix to do? I also have, and highly recommend, the Ninja Professional, but I don't think it's quite a vitamix imitation. It is really powerful and I've made smoothies, hummus, etc.
I also have a Ninja blender and I really like it so far. I haven't made any nut butters but I did make almond milk and was really worried that it would dull the blades but they are still super sharp. I use it mostly to puree soups, smoothies, sauces, almond milk and occasionally I make mayo in it if I need to make a larger quantity then what my stick blender can handle.