1)No sub Categories available. This is a problem for the following reason, If a user is looking to buy a vehicle it stands to chance that they will also be looking to buy parts for that vehicle. Without the ability to have sub-cats part's categories are almost impossible to create.
2)This extension only comes with 1 month of support and upgrades. Support during that time frame is very fast and helpful, normally within 10 minutes. But the 1 month support is an very short time frame and dose not help with bug fixes found after the support period has expired.
3)This extension is not 1.6 ready yet and any information about it's completion date is not easily found on the web site ( also have yet to find the forum for the site yet)
But all of that not withstanding, if you intend to run automotive classifieds on your site this is the ONLY extension to use! No others even come close to this! Great work hope to see more from this extension soon.