I used to be/still sorta am into car audio. I've used JVC, Kenwood, Pioneer, Sony, and Alpine in the past. My thoughts:
JVC: Their stuff is pretty good. I used a similar double DIN head unit in a family member's Scion Tc and it was a great replacement for the factory "Pioneer" that crapped out at 60k miles. I wouldn't hesitate to buy one if the price was right.
Kenwood: Good, solid receivers. I actually just put THIS ONE in my truck last week, and I love it so far. Their audio menu systems can be a PITA to navigate sometimes, but they sound great.
Pioneer: Much like Kenwood in the fact that they are good, solid receivers. Usually on the affordable end of the scale too. Lots of audio options to make any car sound decent.
Sony: I haven't used one in a very long time. The one we had was an early CD receiver, so it didn't have much for sound options. It lasted about 6 years before it crapped out.
Alpine: The most expensive of all the major brands, and the one I've had the most problems with. They also lean toward Apple compatibility, so YMMV. Out of the three that I've had over the years, all three of them failed in various ways: two of them being the CD player dying, and the other had internal electronics problems that had to be fixed by an authorized dealer. They do offer clean sound when they work, especially with a subwoofer, but the audio options are not as robust as others.
I've bought from Crutchfield in the past, as well as Sonic Electronix, Amazon, eBay, Best Buy, and the now-defunct Tweeter, etc and Circuit City. All of those places were fine to buy from. If you buy from eBay, beware that manufacturers may not honor their warranties because they are not considered an authorized reseller. Alpine is especially vigilant on this.
Last stereo I bought (the Kenwood I mentioned earlier) came from Best Buy on sale for $69.99, and I bought the wiring harness adapter at Walmart for $9. I already had a "dash kit" in the truck. I could have reused the harness adapter that was in the truck too, but I was lazy. That Kenwood replaced an Alpine that was failing, and to my surprise, it sounds better than the Alpine ever did. FWIW, my Dakota has a set of Eclipse 6.5's up front and Blaupunkt 6x9's in the rear. There are a million and one ways to adjust the sound on this head unit, and one of them actually takes into consideration speaker size. Pretty cool.
I'd stay away from the "bargain brands" like Boss, Jensen, etc, especially when the better brands are so cheap these days.