Joe,
I really like it. I think it's one of those tools that bridge the gap in the automobile industry. I have seen a stripped down version of this on the Drivetime.com website (only tracks vehicle's from the VDP page) and I have been wanting to add it on my site ever since I saw it.
If you are going to do a "phase 2" of this app I would:
1) Add SMS notification at the users request. (higher open rate) (Twilio has a great SMS api)
2) Add a "follow" like feature for showroom customer with a simple email and/or phone number opt-in. (like twitter)
I have subscribed one of my dealers on Mojomotors.com and the website is entirely based on "following" cars accompanied with a mailing list.
I often markdown some of my cars in my inventory based on the amount of people following them on Mojomotors.com because I know they will be receiving an email notification.(See below)
https://www.dropbox.com/s/7fichkf6jf1iqjp/Image 2013-02-24 at 10.02 PM.jpg
This is such a great tool, can you share usage data with us in the future?