They are not at all bad now, BUT, most will use part of your memory and some of your CPU capacity.
All will be of lower quality than even a low end AGP card, but usable non-the-less.
If you're looking at using the on board graphics with Linux, do some research on the motherboard you're considering and try and find out if the hardware is compatible. Almost certainly, the motherboard itself will be, but the graphics chip may or may not. If I were you, to be on the safe side, I would get a motherboard that has an AGP slot, because if all else fails, you can pick up a cheap graphics card that will work with Linux. Keep in mind, Linux will run beautifully on a relatively low specified machine, so you need not spend a fortune.
Most major distro's have hardware compatibility lists that may help, though sometimes these are not right up to date.