I would be willing to bet that in the proper enclosure, All these "experts" couldn't tell one sub from another by the way it SOUNDS, and isn't that what a sub is for anyway? I have heard a setup using Dayton subs that was awesome. $29 each and without the pedigree. I have a Diamond in the XB and a JL in my Tacoma and they sound about the same to me. Mine are in SQ setups and won't part your hair or anything, but I have heard most of the brands mentioned and most sounded real good. Pick any one and put some effort into the enclosure and I dont see how you could really go wrong.

 

Like others have said, it really comes down to aestetic preference. All of the nice speaker companies make nice subwoofers, its a question of figuring out what you want in a system (SPL, SQ, etc..) and finding a sub/amp combo with the right specs and tuning to sound good in your application.

IMHO JL has gained popluarity due to its styling just as much as its performance/quality.

Since AUdioBahn hasn't been mentioned yet...I'll just say that I'm VERY satisfied with my 2 10s and Everyone I've talked to with audiobahns ALways has good things to say. Great bang for the buck!

 

Perhaps I'm beating a dead horse, but further to my suggestion of the OZ Audio Matrix Elite (http://jti-innovation.com/oz/oz_team.htm) the box you design for the sub is probably just as important, if not more important, than the sub you choose. I made sure my box was exactly 1.0 cuft sealed and tuned to the correct cutoff frequency. Most manufacturers spend a good deal of time and money determining what conditions (box, power, frequency, etc) their sub performs the best at. I've seen and hear many higher end subs that sound sub-par because of the way they were installed or the power given to them. On the other hand, I've also heard the lower end subs that have been more carefully researched and installed sound great.
Another thing I considered when I bought my sub was customer service and product warranty. The people at OZ audio were EXCEPTIONAL in customer service and answering questions. If you go to the link above, I actually talked to owner of that car (he's an OZ Audio executive) and he was the most helpful person I've ever worked with. And the warranty...just try to beat a 3 year warranty...that's a product guarantee!
That's what sold me on OZ Audio (aside from the fact that they are different and you don't see them in everyone's car). I've been into car audio for about 10 years now and I'm not sure if I'll ever get any other sub than OZ. If you want to know any more about the Matrix Elite, let me know.
My 2 cents.