It's been a couple of weeks since I been changing from Phenom II 955 @ 3.6GHz with the lowly FX-4100 @ 4.7GHz and lemme tell you something about it.
Most benchmark tells this is way slower than Phenom II, for example in Cinebench 11.5 my Phenom II @ 3.6GHz got 4.46 CPU ranking, while FX-4100 clocked at 5GHz only reach measly 4.07, same story in 3DMark06 CPU score and some other benchmarks where it lags behind the former.
But in an actual games I felt this is faster, in Serious Sam 3 for example framerate were consistent at 60fps even in heavy fights, on Phenom it will occasionally drops to below playable level. I'm truly surprised! In Skyrim too I thought this CPU will fail, but no, gameplay were smooth as butter. Same goes with other CPU heavy games like GTAIV and Starcraft II.
Still, unless you got a good motherboard, love overclocking or using old Athlon or triple or dual core Phenoms, I don't recommend for upgrade especially when you have quad core or hexa core Phenom II. But if you asked me, overclocking this FX is LOT of fun, haven't have this much fun since old Sempron s754 and Northwood Pentium 4 overclocking where they could reach 1GHz overclock easily

And this is only old Bulldozer based FX-4100, if you aiming for FX line I would recommend Vishera based FX-6300 as a bare minimum for an upgrade.
For Bulldozer, Piledriver CPU owner it's worth to download and install
this.