Right now, most of AMD's 8-core CPUs are priced between Intel's i3 (2 core) and i5 (4 core) CPUs. In many cases even Intel's 2-core CPUs will outperform AMD's 8-core chips because not many day-to-day average user type activities really use more than 2-4 cores, and Intel's cores are 60% or more faster per clock. Intel also has 6 and 8 core chips available, though at much higher cost than AMD's 8 core CPUs. Not only are Intel's cores significantly faster than AMD's, they also use less power to get the same amount of work done, and produce less heat.
AMD has 2 niches, and neither of them is the ultra-high-end:
1) For scientific computing and perhaps video encoding and compression, AMD's 8 core chips are generally faster than Intel's i5's and a little cheaper, making them a good value, though they still don't usually compete with Intel's 4 core i7's, much less then 6 and 8 core models. You'll end up paying more for electricity every month though, and if you live in an area where electricity is expensive, the AMD chip will probably still be more expensive to own overall.
2) AMD's APUs (<$140) have faster integrated graphics than Intel's CPUs. The CPU portion is adequately fast for the typical user and the integrated GPU able to run more games at a reasonable speed than with Intel's integrated graphics. However, there are many cases where a cheaper Intel CPU and a discrete graphics card will be priced similarly (slightly more) but are significantly faster in both CPU and GPU tasks. Therefore, APUs make the most sense where you need something small-form-factor and can't take a full sized graphics card.
You generally don't see many new PCs with AMD CPUs at all, unless that person is simply curious or has brand loyalty.