I've been running my MacBook Air pretty hard in the six months since I received it. It's become my main laptop, in fact. I still use my 17" MacBook Pro, but I'm on the Air probably 80% of the time now, and overall I'm quite happy with it.
In reading postings on forums and various other blogs, I see that some people have been having problems with heat issues and core shutdown during load, especially when an external monitor is in use. I never really ran into those problems, and certainly not to the degree that some have, but I did notice a reduction in framerate when playing back video content with the Air closed and plugged into an HDTV. The issue appears to be related to high CPU core temperatures resulting in one core shutting down, leaving the other to handle everything. This has happened to me, but very rarely. Occasionally, when I have a few dozen browser windows open alongside a pile of other running apps, I've noticed some jerkiness in video playback, but nothing significant. However, in the past few summer months the ambient temperatures have risen and the problem has been more noticeable.
I've read that some folks have had luck in opening the Air and removing some of the thermal grease near the CPU. Apparently, an overzealous assembler may have added too much grease, causing heat to disperse oddly within the system, and adding to the troubles. Since I never had a significant problem with heat, I never headed down this road.
To address my intermittent video playback issue thus summer, I've been using CoolBook with satisfactory results. CoolBook is a CPU management app that underclocks the CPU, reducing heat generation at each clockrate, while still offering suitable performance. It addressed this problem, and also offered me the ability to select the minimum and maximum core clockrates on battery and AC power. This allowed me to prevent the cores from dropping below 1Ghz when plugged in, for example, which made a small but noticeable difference in app launch time, among other things. So CoolBook it was, until Apple released the latest MacBook Air update.
The warning on Apple's Air update states that all software such as CoolBook be uninstalled prior to installing the update, since it addresses these problems, and utilities that mess with the CPU voltage settings could get in the way. I ignored the update for awhile, since I had other things to do with the laptop rather than quit all my apps and reboot. I finally succumbed, uninstalled CoolBook, and installed the update. The difference in heat and performance was immediately noticeable. Whatever CoolBook was doing well, Apple's update appears to be doing better.
The Air now seems snappier, and while it never really got that hot while I was using it, it definitely seems to run cooler overall. The exhaust fan runs less, and at a lower RPM.