This thread is pretty damn heated.
What did you expect?
For some reason, on the internet car forums I've been on most people have enough sense to realize that it's not worth it to talk about politics or religion. But when it comes to money, people lose all common sense and civility. All of a sudden half a given board is considered irresponsible and the other half are the "millionaires next door" and their way is the ONLY way.
There's a lot of black and white in threads about money, but I think that if you know even a little about the topic, you realize that everyone's situation is different. I have no problem if you want to you buy your car outright--good for you. But I'm also ok if you want to finance it. In either situation, it needs to be a smart decision. If your OTD price on a car is $20k, but you only have $25k in the bank, I don't think dumping that money into a car is the right way to go. This day and age, you need to balance having cash on hand with reducing debt (or not getting into any additional debt).
Personally, I have no problem with debt, as long as it's manageable and at a good rate. I like to split my money up between savings, investing for the future, and the various wants/needs of life. Some people are obsessed with having no debt with the exclusion of everything else in their life. A third of my car was paid off as soon as I left the dealership with the keys and I paid nothing out of pocket. I rolled in the value of my previous car into the mustang---and that included paying off the $3k I owed on the previous car when I got rid of it. I'm also almost a year ahead on my payments. So, I'm certainly no where near underwater on the car. And I just bought a house--which I likely would have had to put off if I just dropped my cash to pay off the car.
My debt is pretty reasonable with how much I owe vs how much I make. I wouldn't let anyone on a board like this tell you what the right or wrong way to spend your money is. Trust me, you'll know when you're about to make a mistake. The problem is if you keep going forward.
Oh and to answer your question, I mod lightly, if at all, when I'm making payments. I bought the mustang because aside from the suspension, it's pretty perfect for me as is. There are a couple of small items to change in addtion, but I'll do one or two of those per year. For 2013, I plan on getting rid of the terrible plastic steering wheel that came with the car.