The only attribute is the engine power and the handling improvements for 2004.
So, from a "Driver's" point of view, the car is "Fuel for the soul" as Pontiac claims in their ads.
For the average person, the seats are not as comfortable as the previous generation, and you will find yourself trying to look over the dash. Raising the seat won't help as you quickly run out of headroom if you are over 5'9". It's really a problem if you have the sunroof, which to me opens too short and ruins the car's lines when open.
The "Drive by wire" is a great idea that does not work well at low speeds. The car has a tendency to jerk. You can't feather the throttle smoothly.
The transmission shifts too frequently, which tells me that trans overhauls will be sooner than expected in a car that for the price should have reliabilty written all over it.
Back in January 2004, I traded in a 1995 Grand Prix GTP with the 3.4 twin cam engine, and bought a used 1998 Grand Prix GT (3.8 old school engine) with 57,000 miles on it for $7,000. The car is loaded with Heads-up, auto climate, overhead console, leather seating areas, trac, realtime info center, high power 9 speaker CD system, etc.
Dealer calls me 4 months later and asks if I want to get a 2004 model. I like the new styling and power, so I get a GT2 for $32,000 which includes taxes, financing, etc. Here is where Pontiac got it wrong:
No auto climate, no overhead console because of the sunroof. The info center is now limited to only warning you about a problem instead of constantly showing you Oil Life, distance to empty, real time MPG, fuel used like my 1998 does. No heads-up display unless you order and pay more. Sure, the GTP comes with these things, but for a higher purchase price, higher insurance premiums, and fuel costs. Sorry, a Pontiac is not worth that much.
Third day of ownership, the front wheels went out of alignment. The dealer "fixed it" and the steering wheel was now off center 1" to the right from being 1" off to the left. After two car washes with the car being pulled through, the steering wheel set itself to center.
I bought a black GT2 that had water spots in the finish. The dealer sent the car out to get detailed, but the finish was never as good as an import in black. It was later determined that the paint was never properly sprayed and the "water spots" were imperfections in the paint. At this point, I was not happy with my car.
The door panels on my car squeaked while driving. Drove me nuts. The plastic dash would squeak sometimes too.
The last straw for me was the headlights dancing up and down while driving and the constant plastic parts in the interior squeaking, along with the realization that I was spending $32K for a car that should be well executed versus spending $7K for a car that was just as smooth, loaded, and has a 3 year extended warranty that I did not pay for the 2004.
I took the 2004 back to the dealer, and got my 1998 with all the toys, lower insurance, lower payments, a more comfortable ride, and peace of mind.
The 2004 GT1 and GT2 are not worth what dealers want for them now. I don't know about the GTP Comp G. Too many imports out there to consider before Grand Prixs. The 3.4 twin cam was a better high performance engine than the 3.8 Gen 2 or maybe the Gen 3. But it had reliability problems that GM could have fixed and kept in the Pontiac line-up.