I disagree with Galilee's opinion on the GF4 image quality as many of the units I've tried and tested have been utter crap.
My biggest beef with GeForce cards has been no baseline of quality components as buying, say, a Gainward versus a Leadtek versus an Asus (or whatever)- your 2D and overall signal quality can be night and day between the bunch.
I think the 'reference' design should set a standard for quality and/or ratings of individual components, then 3rd party manufacturers can either meet or exceed this 'reference' standard (or fall short and be unable to earn this 'reference' rating). This will go a long ways in assuring at least some baseline is met, while still giving 3rd parties flexibility in shopping for individual components.
I went through a dozen different GF4's before finally settling for a VisionTek Ti4600, which still isn't 100% as far as sharpness and signal quality, but better than many I installed and tested. On the GF3, the Leadtek Winfast GF3 was the same case, only after unboxing and testing six different GF3's as well. Obviously little to no difference can be determined if one tries hard to clip accuracy- such as only testing at lower resolutions or lower frequency, in non-colorful conditions, or on non-high end monitors. At 1024x768x32 @ 60hz on a Viewsonic/ADP/Relisys, with Quake2 there is no discernable difference. At 1280x1024 @ 100hz, on a Sony or NEC with Morrowind or Dungeon Siege, the difference is night and day.
As far as rasterization methods on the chipsets themselves- you can argue this until you are blue in the face. The fact of the matter is ATI and NVIDIA chipsets do things differently. Mipmapping, texturing, stenciling, alpha blending- they are very, very different. Subjective opinion is what reigns at the end of the day, but sane/fair/non-fanboyish monotony can easily pick out the differences. IMO, alpha blending in OGL is very clean on the GF4 yet ugly on the 8500, as with mipmapping in D3D is very clean on the 8500 yet ugly on my GF4. You can pick out, case by case examples that illustrate the different methods these chipsets use for various effects so there is no sweeping "XYZ is bettah!" since both have different methods and different goals in mind.