Never really splurged on anything truly silly. I did get an upgrade from a "regular SUV" class and ended up with a Ford Explorer Eddie Bauer edition for two weeks. That was nice, it was like driving a sofa and seemed to have a huge amount of power.
I have sometimes considered renting something a bit silly for a short getaway but find I prefer driving my own car, where I know how it handles and how to manoeuvre it, and where I know I have space in the back for everything I want. I've had a few issues in the past with renting a larger vehicle because I needed to haul stuff and then having to repeatedly tell the guy behind the desk that a vehicle half the size of what I booked isn't an acceptable alternative. Yes genius, I rented that enormous thing that does 12mpg for a reason, and not because I have shares in the oil companies.