My 20 cents:
TL;DR: In the short term, it's the best time to buy. Long term? Wait until next year, or maybe longer...
I do agree with john123: the most significant gains for this generation are an extra hour or two of light-duty battery life, and a $200 price drop. The rest of the updates, while nice to have, bear little to no significance on the day-to-day user experience for most people.
That said, I found a section from AnandTech's review of the 2012 Retina somewhat revelatory. In Anand's view, seen from 2012, Haswell was going to be a very significant upgrade in terms of GPU and CPU performance. The proverbial "tick" to Ivy Bridge's "tock". Well, it looks like Intel's clock might be in need of service since it just "tocked" twice in a row...
There is evidence to suggest that Broadwell may deliver the goods in a more meaningful manner, but it also might not. It could just as easily turn into another, routine 10-15% performance boost. We can make educated guesses all we want at this point, but until those CPUs and GPUs (and nVidia's new GPUs) are produced and installed in a MBP, we have virtually no way of knowing for sure.
IGZO displays are a game-changer in my opinion, but there too, with recent rumours putting them in iPhone 6, it's a safe bet we won't see an IGZO MBP until next fall at the earliest...
Bottom line: I needed a new computer now. I have a friend who currently works at an Apple Store, which might not be the case next year--can't look a gift 15% discount in the mouth.
And so now I get to proudly wear a t-shirt that says, "I own the fastest MacBook Pro in existence"... for the next five months, at least.
I didn't intend to upgrade now, but I'll embrace it. And if next year's update leaves my machine in the dust to the extent that it bothers me, I'll upgrade again. (I have a funny feeling that won't be the case though...)
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Now, is there a difference between the 2.3 and the 2.6? Speed wise? I'll be doing 3D work and some pretty light photo and video editing. Well, video editing may get to the medium to high side once in a blue moon. I'll be upgrading my desktop to a custom built desktop next year with the broadwell chip set so I'll be putting big projects on that. But I will be using this laptop to get started on projects between classes. 2.3 should be alright?
I sprang for the 2.6GHz 'cause I'm kinda OCD and the extra $180 was worth it to avoid potential buyers' remorse. Fortunately, at the end of the day, it turns out I had no sentimental attachment to that money.
And based on the benchmarks I've been seeing the difference is closer to 7% overall between the 2.3GHz + 2.6GHz. Really nothing to write home about.
(I probably wouldn't have gone there if I wasn't getting the friends & family deal.)