I disagree with some of these.
Not really. A laptop as fast as a typical mini-ITX 1.2Ghz set up can be had for about $400-$700, especially if you wait for good deals from Dell, etc.
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Often require low-efficeny inverter is no DC-DC transformer available.
Not really. You can easily find DC-DC auto/air adapters. Kingston makes tons of these. They run for about $30-$100, depending on how much power your laptop demands. I bought mine for $15 shipped off of eBay and it's been running just fine.
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Limited expandibility
I somewhat agree. However, most things can be hooked up via USB/serial now. An mini-ITX board only gives you one extra PCI slot (or 2 if you use a riser card) anyway. Not really a big deal in terms of expandability.
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Hard to get "boot on power"
Definitlye agree with this. Getting the laptop to power on when you turn on the car in the biggest pain in the ***. In most case, you'll have to solder wires onto the board and have to devise a relay circuit to turn it on. On the flip side, installing a "shutdown controller" is as easy as about 10 mouse clicks in Windows XP.
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Docking station may be needed
Wrong. Docking station is just an additional convenience that a desktop set up cannot have. Bringing your CarPC into your house to work on it is as easy as pushing two buttons if you have a docking station. With a desktop, you'll have to unhook all the wires and then re-hook them back later. I don't know why you even listed this as a laptop "con." It should be a "pro."
Of these cons, only #4 is really a big deal.
Some others you didn't mention.
Hooking up an external screen to a laptop. First, most laptops do not support 800x480 so if you have one of these screens, you'll have to use 800x600. Second, most laptops do not display boot messages and the windows bootup screen on the external screen (only on the laptop's own screen), so if your system hangs on bootup, you'll have no idea what happens. This might be a pain in the ***.
Sound card. A laptop's built in sound processor sucks. Period. You'll either have to use a PCMCIA card or a USB card to improve on this. If you're lucky and you laptop has SPDIF out, you can use an external processor.