Just want to see how does it impact the performance and MPG. 
I'm driving a 04' automatic crossfire, travel about 50 miles daily, 20% local, up and down the hill, and 80% freeway, around 70 mph.
I have been getting about 23.3 MPG, using Premium 91 octane Top tier gas (FYI, I always use Shell, occasionally 76 and Chevron). I just switched to regular 87 octane gas, probably for the next two months for a more accurate result. I expect a less MPG, but I want to see the actual "mile/$". 
RESULTS (updated Feb 3, 2008):
After a few tanks of regular gas, I got some results and driving experience that I want to share with everyone who wants to know.
Gas milage:
with top tier (Shell) Premium 91 octane gas, I get average about 23.5 MPG (ranging between 22.5 - 24.5 MPG depends on driving conditions). During the 5 tanks of top tier (Shell) regular 87 octane gas, I still get average of about 23 MPG. (First tank 23.9, second 23.3, third 22.6, 22.5 and 22.7 MPG) I found the data quite interesting, I'm pretty much getting the same MPG on both premium and regular gas, difference is minimal.
Performance and driving experience:
On the first 100 to 200 miles switched to regular gas, I can feel the engine running a little rough at idle, but I don't feel that anymore after the first 200 miles, well, maybe once in a while but that's is normal. I think this is the time while the knock sensor and computer gets to work.
Regarding the horsepower, I don't have any equipment to measure, but on my regular driving experience, I don't really feel any difference, it drive the same way. But I have to point out two interesting feelings: 1, sometimes I feel the lack of power with regular gas when going on high RPM. 2, I think the up shifting is delayed with regular gas (mine is an automatic). But I don't know if this is real or just my imagination, since I know what gas I have in the tank and so my psychology may affecting my sense.
Does regular gas do any damage?:
I did some research and found there are two theories. Therory A says that low octane gas used on high conpression engines will cause early explosion causes knocking engine, the wrong timing and extra heat energy may damage the engine. Therory B suggests that new engine like ours have knock sensors to retard the timing and the engine will run with no problem. I believe in both, and my suggestion is if you drive like an old man, regular gas would works on the crossfire, but if you drive it like a sport car like wht it supposed to be, go with premium gas.