In the office, I use an NVIDIA GeForce 9400GT; at school, we use ATI Radeon HD2600. What's wrong with this picture?
Nothing much, unless you consider that in both cases I have to install the proprietary drivers when I'm using Ubuntu. For the most part, they work, too. So what could be the problem with that?
Well, whenever I upgrade the kernel, as happens with the Update Manager, the graphics breaks after the reboot. Either I'm left with a low-resolution screen, or worst case, no screen at all. This is because the proprietary graphics driver is tied to the kernel, and vice versa. Changes in the kernel will not automatically reflect the graphics driver. Hence, a reinstall is needed.
After a kernel upgrade, I find it helpful to boot into recovery mode and reinstall the graphics driver (therefore, don't delete the graphics driver; keep it handy on disk). It takes all of five minutes, after which I can just resume booting. Graphics should work fine after that.
Still, it is a bother. But since I want some gaming goodness out of my machine, I'm stuck with this less than optimal solution.