I've never upgraded from my second ed and won't now. Whilst it was very well regarded 25 years ago, there are now better books for N America IMHO.
It may be that so critical an assessment will have the last word in this thread, but for those contemplating one single guide for North America it might be worth noting points in favor of the NGS 7th edition (based on experience with the preceding six editions):
Plates with facing-page species accounts & range maps, with confusion species often on the same plate or in close proximity.
Reasonably faithful renditions including many subspecies, seasonal plumage variations, and juvenile birds.
Concise species accounts noting salient ID points, voice, range & habitat. The plates are also annotated with the most significant field marks.
Comprehensive in scope, depicting all North American species accepted for inclusion in the most recent AOU checklist.
Updated taxonomy (splits, lumps, etc.), per AOU.
Portable.
Others might well add more benefits. Some might dispute many I've enumerated. But NGS may well be the dominant field guide in the North American market. I own (& use) many of the alternatives (Sibley, Peterson, Kaufman, among other). I have high regard for each, but likely as not I'll try to solve any ID puzzle first consulting NGS. It's always in my field pouch, along with one other from among those alternatives.
The only volume mentioned by title in this thread as superior is the Collins guide for European birds, and the Collins NA guide doesn't elicit similar praise.
Gary H