Bought my Z a month ago, so just spent a few months test driving cars...
What everyone else says, it just depends on the dealership and the salesperson. Some treat their potential customers better than others. I don't think you can really say that BrandX dealers are great or are terrible, because it just depends. I've had good & bad experiences at Ford & Chevy dealerships.
My local Ford dealership is well-known for being jerks. No idea how they stay in business but they do. Ford dealership one town away is usually recommended as "nice", but the few times I've been there it's mixed; sometimes ok, sometimes not. Very pushy though. Recently drove a Mustang GT at a totally new-to-me dealership and they were great. No hassle to test drive and the salesman was one of the nicest and least-pushy car salesmen I've ever dealt with. So, it just depends.
Worst dealership experience I ever had was at a Lexus a few years back. Shopping for $30-$35k sport sedans and wanted to look at an IS. The dealership wasn't busy but we were told by their "sales coordinator" that we'd have to make an appointment to look at one beyond what was sitting in the showroom. And furthermore, that they'd need to run a credit report before we could test drive one. Uh, no thanks. That one person has destroyed any chance that I'll *EVER* buy a Lexus.
Oddly enough, the Acura dealer across the street is part of the family of dealerships, and they had no hesitation at throwing me the keys to a $40k TL-S that afternoon. Go figure.
One of the better experiences I've had is out our local BMW/MB/Porsche dealership. I've not had the chance to actually buy anything from them, but they've always been extremely professional & very pleasant to deal with when we've been browsing. Not pushy or snooty at all.
How you look matters. It shouldn't, but it does. Dress like you're going on a job interview or out to a nice dinner, and you'll get a lot more respect than you will showing up in cut-off jeans and a dirty t-shirt.