IPC is down in many uses, so most of the time it has to be made up with more cores/frequency.
The saving grace of Bulldozer is that it has 8 cores that it can use on highly threaded tasks, which it trades blows with a 2600k, but that is no where near good enough.
Will everyone here always be using 8 threads at one time? No. That is the problem with matching a 2600k with their threads; they won't always be used. When the threads aren't used, it loses to even the 2500k. It even loses to a 2500k while using all 8 cores in very branch heavy tasks.http://www.anandtech.com/show/4955/the-bulldozer-review-amd-fx8150-tested/5
Being so big and so power hungry doesn't help it at all. The 2600k/2500k can overclock to Bulldozer levels, not only due to a better process, but Bulldozer's design just puts out so much heat.
2 billion transistors barely matches 1.1 billion on Sandy, and Sandy includes integrated graphics. When SB-E comes out, Intel will beat Bulldozer in single/multiple threaded tests with less transistors and equal TDP(unless those 180W rumors are true). Luckily Intel most likely won't price SB-E six cores in Bulldozer's price range, since the high end would than compete with the low end Sandy.
AMD designed Bulldozer to compete with the mid range Sandy Bridge. Does it compete in my eyes? No. Bigger die, more power, and it's slower in most cases for desktop use; sure, it can also come out a little bit faster than a 2600k, much like the Pentium 4 v. Athlon 64, but that's not the case most of the time.
I can't recommend the 8 core for much of anything at this point. It can outperform the old Phenom II arch, but it can also underperform it to a point that even an overclock v. overclock might not make it faster than Thuban. If the 4 core is at least decent compared to dual core SB systems, I will at least be able to recommend that.