It's actually not that hard at all to put 5.1 in your car, there are very few problems with the 5.1 or 4.1 as most people skip out on the center channel.
The difference is like mcDonalds cofee vs Starbucks.
With Mp3's you'll hear the difference when tuned correctly. The discerning ear can do this easily, one who doesn't care, most likely will not notice the difference.
But if I could break it down for you with one question it's this: Do you want stereo 2 channel sound coming from all your speakers using left -> right channels. Or would you like the sound to be 3 dimensional driving each speaker with seperate sounds?
It isn't perfect surround sound but with today's DSP's and 5.1 hardware in computer cards, it's very good.
Everybody usually discards information like the above as it's too small a space to make any difference. I'll give you a lessen in sound and you can tell me from a mathematical\physics perspective that I'm wrong, your ears may deceive you but math and physics always tell the truth.
Consider this, with 2 channels left and right and you have 5,6,or 7 speakers. There is a considerable overlap from front and back speakers (some people call it filler, it's more like sound traffic). With same sounds coming from front to back and back to front you get a certain amount of cross cancelation. Using crossovers solves some of the problem but not a lot. In order to get true clarity you need to have 4 or more channels BEFORE you send it to the crossovers to deal with.
ALL amps and crossover hardware is geared toward separation of sound...this is there function. But why not help it along before muxing sound for it to sort out when you can do it via the DSP chipsets in the computer? Then your seperate sound goes to the individual channel and sound clearer and crisper.
For the above reason alone it's a great thing. Now for cool sounds wizzing around your head arguement many people think the small space is too small for anyone to notice that. When in fact this is mostly untrue. People's ears and sound space even in a yugo can benifit from surround sound. You will hear sound wizzing around your head if you have a true surround sound decoder and source such as DVD's sound track. If you've designed your sound system well, you can even get a surround sound card that will emulate and split out the stereo source and upsample it to a surround soundish output. This kind of hardware is growing in features and is coming down big time. I can say that most 6.1 and 7.1 and some higher end 5.1 chipsets are producing a quality that is satisfactory for most audio aficianato's.
Lastly to top it all off 80% of the people that have said you won't notice the difference have not done it to actually find out. Or have not sat in the car long enough to figure it out. Number one reason why this isn't in large done or done properly? It's a hell of a lot of work!
You might be asking if your still reading where's the math and physics? Well it's mostly in the building of the design where you'll see that work, I'm a bit of an engineer and anyone who's seen my rig and heard it know that I'm good at it and they have all noted my sound is amazingly clear.
I do this on a budget. My rig cost me under $1500 for the whole system and I do it with garage sale parts

meaning someone thought their speakers were crap and threw them at me for measily $100. Well most of the equipment is great as the technology in the early 80's and 90's still works well especially when you've got a computer as the back end driver.
Note this everyone...an AMP will take sound and amplify it even a ****ty one will do. But some of these guys that did sound competition's in the 90's are selling there rig cheap. And the higher end amps like alpine and PPI are great to use. The computer's DSP's bring the whole system a new life that no head-unit sub $1000 will do. This is my opinion and I've proved it.
I have a sony XEC-1000 old school crossover, a Rodek 2150i AMP monster amp and pumps out hard! And old *** Acustic sound 10" subs, I'd get rid of them if they didn't sound so good right now...they're probably worth $2 on the market today. I've also invested in a 4 channel Alpine amp to drive the other 6 speakers. The rest of my system is a stock RSX type-S sound system (6 bose speakers set). Runs nice and hard and I can't complain about a thing.