I've been using the Asus EeePC 900HA for a couple months now, and I am still quite satisfied with my purchase. As with any other electronic device, there are changes that I would appreciate in a future replacement, but they don't make the netbook any less usable. I have used it on pretty much daily basis for almost 2 months now, and it has stood up to that use, and more. Most of my use currently is being done outdoors, so although cooling has not been a problem, exposure to dust and other particulates has.
Like most plastic devices, the EeePC is a dust magnet, and I have not been able to keep it clean for the life of me. Another factor that has been bothering me is my inability to turn the touchpad off. Since I am using a wireless mouse with the netbook, the touchpad has only served to cause typos when my thumb inadvertently touches it and moves the cursor in mid-sentence. The battery life has also been much less than anticipated or claimed by Asus. At most, I'm able to get 4.5 solid hours of use from the device, under low load conditions, mind you. Personally, I count on 3 hours of actual, everyday use, with some reserve power to save files and shutdown the system adequately.
Other things I've noticed include flickering of the backlight at the lowest setting; as I've mentioned, I often use my netbook outdoors, and frequently at night. The second lowest brightness setting is more than required in these circumstances, but that is when the flickering is the most obvious. A more annoying bug has been the reconnection to a wireless network after coming back from a suspend mode; about once out of three times, I need to reboot the computer to get the wireless to work again. This last issue might have more to do with the operating system (I've been using Eeebuntu with the Netbook Remix as my OS) than the actual hardware, but I have no real way of confirming or denying such a claim.
I will continue to post more information on my experiences, but so far, I am quite satisfied with my little machine.