OCC has published a review on the AMD Athlon II X2 255, X3 440, X4 635, Phenom II X2 555, and X4 910e. Here's a bit:

With the testing done, we can see that the new versions of the Athlon II and Phenom II processors offer some of a performance increase, and if you are looking to upgrade from the less core processors to the higher core count ones, then I suggest to go ahead and spend the bit more to get these newer versions for the increases they offer. If you are just looking to trade up say triple core to triple core, then the differences are not worth the money when what you have is good enough. The Athlon processors offer the best value when it comes to workstations or every day computing systems, where you may need a bit of power for some tasks but do not want to lay down the cash for a high end CPU. These would also include home theater PCs when paired with a nice motherboard, such as one with a 785G chipset. The best bang for the buck goes to the AMD Phenom II X2 555 processor, which during the testing I was able to stable push it above 4GHz with very little effort. The processor never reached its thermal limit and remained stable for a nice period of time with no issues. The bummer of the bunch was the Athlon II X2 255, which I could not overclock even a bit no matter what I tried. It did, however, perform on par at stock but, hey, we are enthusiasts and do not settle for just stock speeds, right?

Overall the new processors offer a great price to performance ratio and I would recommend them to anyone in the market for a new processor. I would not say go out and toss away an already good processor for a small performance gain, but for a fresh start these are a good buy. If you are on a limited budget and want to overclock, the best out of what you got for $99 the Phenom II X2 555 will offer you great performance on stock and even better when overclocked. With the new silicon used for this processor, you should be able to get above 4GHz often allowing you for the best price to performance out of the bunch.