I used to think that the 2000 restyle of the Bonneville was inspired by Satan himself. I hated the look of the SLE and SSEi. To me, the 96-99, was the best. Then along came the GXP. That completely changed my mind on the styling issue. I now like the look of the GXP much better than my SSE. Performance aside, the GXP has a more updated look. All the body cladding on the 00-03 SSEi'* given them a mid90'* feel. (Yes I know that there is some body cladding on the lower half of the GXP, but not the bubbly tupperware type). The fog lights on the SSEi'* were acceptable on the 92-99s and actually looked good. On the 00-03'* they look like they're hanging off the bumper. That'* fine if they weren't the size of dinner plates.
As for performance, I don't have an SSEi but my GXP has the best handling of any Pontiac I've owned. The ride is far superior to the SSE even when it was new. The SSEI might be faster, but the Northstar is more reliable. I've read here that the SC 3800s aren't susceptible to the UIM problem, but really, they aren't? This Northstar in my GXP is simply the smoothest engine I've ever owned. And I always thought that the 3800 was the best.
Add to this what Venom wrote
And uniqueness of low production numbers
and you've got a good overall package. I can't tell you how many times people have taken a second look at the car and asked about it (and I've only had it + 2 months). I always hear "it just doesn't look like anything Pontiac".
By the way, the EPA lied on the GXP fuel economy. I drive alot in a hilly winding city environment and my fuel economy reads "20.2" this morning (I do use premium). I know that the 17 is just the center of a range, but I have consistently gotten 20-26 city/highhway with this car.
One downside for me. I'm not sure what seats the SSEis have , but the corvette seats in my 96 SSE are far superior to the flat, uncontoured leather/suede seats of the GXP. I am considering finding the corvette seats and replacing the pine boards they call seats in the GXP.
For me, though, the key diffence is styling and (sorry SSEi owners), the GXP is just far better looking.