I use my Vitamix at least once a day, and often several times a day. I think I've already used it about 5 times this morning, and that's just for me -- I don't have a big family and didn't have a bunch of friends over for a brunch party or anything today.
I agree that it's not very useful if you only make an occasional margarita or something, but it's great for so much more than drinks/smoothies. If you've only ever used a "regular" blender, you probably would never think to do with it many of the things a Vitamix can do.
In my case, it makes cooking at home more pleasant and healthy because I use it for sauces, salad dressings, soups, nut butters, home-ground flour, non-dairy milk, date paste (substitute for refined sugar in many recipes), banana soft serve "nice cream," etc. I probably use it for about 80% of the recipes I make. It allows me to DIY a lot more of my basic ingredients (thereby making them cheaper and usually healthier than the store-bought option), and it provides a better (usually much, much better) final product than a regular blender would. Whether the savings on homemade staples and on eating in since the food is so good at home actually balances the cost of the Vitamix over time... I haven't done the math, but I suspect that over the life of the Vitamix, I'll come out ahead.
Putting aside the above, I spent $300 on my refurbished Vitamix a few years back, with a 5-year warranty, but I suspect it will last as long as a new one, which comes with a 15-year warranty. That's $375 worth of $25 cheapo blenders replaced yearly, which is about how often my old blenders tended to die, with obviously much less use than I'm currently putting on my Vitamix.
If the financial argument is not convincing (I admit there are a lot of "ifs")... I like it. I can afford it. I don't spring for the most expensive option all the time, but in this case, it enhances my life enough that it's worth it to me. Isn't that what Mustachianism is about -- making your dollars work for you to give you the best life possible? A Vitamix would probably be meaningless in terms of "life enhancement" to many others, and that's fine -- they can spend that $300 however they see fit.