I've just assembled a new gaming rig with this CPU and as an AMD addict for years (decades actually), it's the first time I am not blown away by one of their new processors. I do now understand the mixed reviews the FX has gotten since its release. Now don't go thinking this is a bad CPU. It's not. It's just not what I expected it to be I guess.
First of all, the FX 4100 is a really "open" CPU, it lets you toy around with a lot of its features and options thanks to the fact that it's unlocked. People who are a little more than just computer literate will love this, cause it enables you to tune your processor the way you want it and explore its limits. AMD has done a very good job on this side and provides the tools to do just about anything you want. If you own a good motherboard (and you should with such a processor) as I do (Gigabyte 970A-UD3), you can really unleash some of the hidden potential of the FX.
The other interesting thing about the FX is that it's a semi-intelligent CPU if you will. It adjusts itself to use more or less power to execute certain tasks. Or so it should. The problem is, it doesn't seem that the software really understands that. Sometimes, you will get very good results performance-wise and then another task will just behave so poorly that it'll make you regret your old Athlon or Phenom. Another issue is that certain games for example, do not want to run at all with an FX installed. Red Faction: Armageddon is one such game. This is quite sad and honestly, it feels like the FX was just released a bit too soon, with lots of promises to wipe out the bugs at a later point in time.
Overall, the FX is not bad at all, just not ready to hit the market, as too many developers still code their applications and games to use a single core, whereas AMD encourages buyers to go for up to 8 cores where their CPUs excel in performance.
I do like the FX for what it is, namely a true step forward in CPU evolution, but at present times, if you are a gamer for example, you might want to stick to the Phenom family for a while until the software evolves with the hardware.
UPDATE: Gigabyte just released a new BIOS update which includes the new AMD AGESA instructions set and it fixes, well, pretty much all the issues with game compatibility and gives a nice little performance boost. Thumbs up for AMD on this one for this quick fix.