Maybe you're .... uh .... holding it wrong? Oh, crap. Wrong forum.
Are these wifi issues very common? If so, a return might be warranted. If not, an exchange for a new device might be better. I'll usually give an electronics product at least a second chance. We bought a $900 Kodak digital camera years and years ago. It was like a revolutionary 1 MP or something. What a beast! Anyways, the first one, brand new from the factory, did not work properly. Lots of issues, times when it wouldn't take photos, etc. They sent us a new one to replace it. Kodak typically had, and still has, very good customer service. But the same proiblems happened with the replacement unit. Still being under warranty, they sent us a replacement once again - but this time, not a new camera, but a refurbished. All be danged if that thing didn't work like a charm, every single day, every single shot ... PERFECT!! ... until that unfortunate moment when a clumsy classmate ran into me and knocked it right out of my arms during a school project (our camera was substantially better than my school's). I looked down just in time to see it hit a concrete floor lens first.
Anyways, I've had good luck with refurbs and display models in my time. Another example is with laptops. I bought a brand new Macbook 13" straight from Apple. It was a second gen with a Core Duo, but not Core 2 duo. Right out of the box it had problems. I paid extra for the faster processor and actual DVD burner, and it was the most that I'd ever personally spent on a laptop, or any computer PERIOD. Bought it for college. The keyboard and two other internal parts were bad right from the get go, and I had to send it in for repair. Then, within a year, the battery died - not like the duration of a recharge went down, it just DIED, as in a big red "X" over the top of the battery symbol. Unplug = hard shutdown. They replaced the battery. Then the DVD drive died because of a firmware update. Then the second battery died in similar fashion to the first.
Then, while I was taking notes on this same white Macbook in class, it started on fire. That's right. It was plugged in and I was taking notes - no background apps running, so no demand on the device - and we smelled smoke during a lecture. I looked down, and flames were coming out of my power adapter.
In contrast, a Sony Vaio I bought at Best Buy - not only a clearance model, but also the display model - is still kicking to this day. It's over 5 years old. We got a replacement battery under the service plan. That's it. The original battery will even last you about 30 min. if you needed it.
My 4th Eris (like new factory replacement under warranty) finally works well for the wife. The brand new one had a dreadful speaker failure within a month.
Wow. I don't know why I wrote such a long story to make such a short point - If you're having serious wifi issues, which you should be able to reproduce consistently, or at least in front of a tech a time or two, a second try with a different device might be in order. I have a lot of faith in Google's selection of the Nexus S for their flagship device. I would expect them to have put several test units through the paces. My Fascinate does not have wifi issues. It's reception is not quite as strong as the Droid X I used to have, and it's not as strong as my new laptop which has ridiculously good reception, but it's as good as any smartphone I've personally used, and it has remarkably similar internals to the Nexus S. I'd give it another go with a fresh device.