JL Audio is one of the highest quality car audio manufacturers around.
Their subs provide the best compromise between sound quality and sound pressure level you can get. The 10-W7 is the best sounding sub I've ever heard and almost the loudest as well (louder than a pair of 12-W3s and plays as deep as a 15) and is nearly indestructible.
Their component speakers have gotta be the best sounding car audio speakers you can get for the money. I'm sure there are some others out there that sound better (that are not just piercingly loud like MB Quarts, PUKE) but they will cost way more for a difference in quality that only someone born with very good hearing who hasn't ruined their ears yet could tell.
Their amplifiers are absolutely killer.. just read the product information about them.. it speaks for itself.
I have gone through many car audio systems before and my current system is an entirely JL audio system (except for the head unit since they don't make those, heh). Right now I only have the front components and rear door coaxials and the head unit in my STi. My amps and 10-W7 sub are sitting in the house because I'm just conflicted about putting them in for a number of reasons:
1. I don't want to weigh down the car.
2. Loud subwoofers always eventually make your car permanently rattle and squeek like a son of a b (and this car is just too sweet to be disrespected like that).
3. The car's body is so thin and light that a subwoofer would make it buzz and flap so bad that you couldn't stand it.
4. The STi is one of the worst cars to try and do good sound quality in since the sound deadening in it is so weak.. if you're sitting still: sure it'll sound great, once you're on the highway you can't hear a thing. Of course you can deal with the sound deadening thing by adding more of it (dynamat or similar) but that just goes back to my problem number 1.
I love my music loud and clear but I won't compromise my sweet car to get it. I am considering doing a single JL 8-W7 in a sealed enclosure with a five channel amp.. but for now the thrill of just driving the car is more than enough to entertain me.