I definitely believe it's worth the upgrade if you're running an old AMD Athlon 64 X2 or an Intel Core 2 Duo, but not if you're already using a Phenom II x4 955 processor and needed to upgrade both the MB and CPU because the 790GX chipset don't support bulldozer, like I did. A lot of the people at Tom's Hardware recommended the Phenom II X6 1090T as a cheaper alternative, which performed on par with the FX-8150 in many of the FPU benchmarks.
I did some more testing and I think I have figured out that the bottleneck is AMD's shared pipeline. Since the CPU has four pipelines and 8 integer processing cores, 2 cores share one pipeline. Through some experimenting, I have concluded that the cores are arranged like this: (0,4) (1,5) (2,6) (3,7) with (X,X) being a pipe.
I did some testing by changing the affinity in Windows Task Scheduler and limiting Fractal Extreme to four cores, and finally two.
With affinity set to four Cores, (0,1,2,3) was about 20% faster than (0,2,4,6) and (0,1,4,5). (0,2,4,6) and (0,1,4,5) were nearly identical.
Limiting myself to two cores, I shrunk my test render file to 200 pixels square and set affinity to cores (0,1), (0,2), and (0,4). (0,4) was about 20% slower than (0,1) and (0,2)
Further complicating the issue is that the threads tend to collide with each other when they are on the same pipe, but if the threads on different pipes share the same data, there is a higher latency when fetching said data, so threads that make a lot of data calls might actually perform better on the same pipe. That said, the performance gain, at least for integer calculations, is probably much higher per the additional cores, than Intel's hyperthreading, which creates an additional "virtual" core for every real one.
Because Windows 7 task scheduler is ignorant of the dual core pipelines or turbo core functionality, if you use a software app that only utilizes four threads, you may want to manually set the affinity to (0123) or (4567) so that each thread gets it's own dedicated pipe. Or you could also try setting even cores (0,2,4,6) or odd cores (1,3,5,7) so that the four threads are shared across two pipes, which would enable turbo functionality. Experimentation would reveal which setting is fastest, and it would probably vary per software.
If anyone has access to a Sandy Bridge 2600k and Fractal Extreme 64-bit, PM me so I can send over my Mandelbrot deep-zoom test file (728 zooms). I'd love to see how the FX-8150 stacks up to 2600K with just pure integer math (mostly repeated 64-bit multiplication). My test file got 2m48s at 400x400 pixels in FX v2.20 (on all 8 cores, OC @4.2Ghz). I think integer maths was really the one benchmark area in which the bulldozer really blew away the Sandy Bridge.