It's like most advertising since the modern version of the idea of branding slowly took over advertising in the early 20th century, and definitely since radio and television solidified this practice: the individual commercial doesn't convince you about anything, but the group of them together make you A. aware that the brand and product exist at all; and B. understand that the brand connotes some set of ideas (luxury, value, engineering, beautiful design, good taste, etc.).
Basically it's an active strategy built into a matrix between market analysis and product development. The problem arises when the ad men just focus on market analysis and ignore the product development, or worse, let the bean counters take over the development– i.e., just focus on building an image without worrying about the substance. It's a lot easier than making sure the product actually lives up to the hype. I actually like old Cordobas, but whoah they are not what Ricardo Montalban would like us to believe.

I think that they are compelling, as long as they make an impression. I can think of a handful of car commercials that, even now, make me want to own the vehicle that is/was featured in them. More than one of them is of a truck bouncing through mud/dirt in slow motion. One of the more recent ones was during the holidays when there was a Dodge Challenger slowly circling, and doing a burnout, around a litter of other Chrysler vehicles. That one made me want to buy a Challenger!

If I based my automobile buying off commercials I don't think I could own a car. Especially with commercials like "Imported from Detroit", "Watch Me Drive Through Mud" and "I'm Driving Through A City". However, I did own a VW beetle and they had one of the best and most compelling advertising campaigns I've ever seen. The VW had some great "ironic" print ads and some really good TV ads. I also really like this ad. I think I could buy a Volksie bus from this ad or from others in it's series of South African VW ads.
[youtube KpRhlCMGwcg http://www.youtube.com/watch?v=KpRhlCMGwcg youtube]

Maybe informative, but usually not THAT compelling. It is a good way to find out what kind of rebates might be going or that type of thing. It might get me to go to the net to check something out.

The only vehicle I have ever owned with a memorable advertisement is my 1998 Frontier. Back then, Datsun…err… Nissan was running a series of commercials with a Yutaka Katayama looking actor telling us all that "Dogs love trucks". However, that was not a part of my buying decision. No, it was Nissan's reputation for building bulletproof small trucks that made me buy it. I figured I could push it to half a million miles without an overhaul. So far, 362,000 on the original engine and owner, so I have yet to be proven wrong.