It would depend on the workload. If you were doing a CPU intensive task, or playing a CPU limited game, the difference would be obvious. Under light load, the difference might be less obvious. But that is like driving an economy car vs a powerful truck. Both would be fine for cruising a flat freeway at 60 MPH, but if you want to carry a heavy load, you definitely need the power of a truck.
In any case, I will still take objective performance data over casual observation any day.
Haha, except in this case that powerful truck is actually more fuel efficient than the economy car, but you make a good point nonetheless.
I had the opportunity of having my friend over for a week, and we setup both our computers, each with two 23" monitors, on a 6ft table. Yep, it was tight. His computer is a Phenom II 970 w/ a 550 Ti, and my computer is the 3770k + 285 listed below. I did have my 5850 in my 3770k for a bit, but being that it's the only video card I have with an HDMI output, I mostly use it for HTPC purposes 'cause my TV doesn't have a DVI input.
We mostly played Planetside 2 and some Starcraft 2, but you could really tell the difference in Planetside. Not that I'm particularly shocked by that, but it does reinforce the fact that depending on the workload, you may or may not notice the difference between a C2Q and a Haswell rig. But play the right game and you certainly will. I played Planetside on my brother's secondary PC (C2Q 6600 + 560ti) when I was visiting my family a few weeks ago, and it was darn near unplayable in certain areas. My 3770k at home on the other hand, even with the 285, never flinches.
As others have stated, everyone's workload is going to differ. I tend to play mostly older games as of late (except PS2), and realistically, the power of my 3770k has gone to waste. We're talking games like HL: Black Mesa, System Shock 2, and Starcraft 2. With the exception of SC2, I don't think I'd notice much a difference playing between my 3770k and my old C2Duo.
With all that said, I'd rather have a powerful CPU + a weak video card over having a slower CPU and a powerful GPU. In the games that really do stress your CPU performance, the extra CPU power is very noticeable. When we were flying around Liberators in PS2, I was getting nearly double the fps than my friend's PII-970, and PS2 doesn't appear to utilize HT cores according to my CPU meter gadget.
When I transition to playing games that are more GPU limited though, I'm sure my opinion will change lol.