I have a Nexus One, I ordered it on day one and haven't had to exchange it at all.
It's rock solid, fast and a great phone!
That being said, I love the concept of a dedicated GPU on the Vibrant as well as the fantastic picture and video quaility.
Same Megapixels, but the shots look so much clearer and the colors are more true coming from the Galaxy S phones.
As others have said, the community is great for the Nexus One, but I have no doubt it'll be just as great for the Vibrant.
I don't think I'll switch FROM the Nexus to the Vibrant, but if I had another phone and had to choose, I'd probably choose the Vibrant.
Phone: T-Mobile Galaxy S IV | ROM: VirginROM (Google Play Edition) | Launcher: Nova Launcher