Nexus phones are sons of lost. Nobody, neither the manufacturer, the carrier, nor Google take care of them.
First, these phones are guaranteed to be buggy, because the manufacturers won't put enough attention to them. They are Google branded phones, why Samsung or HTC would care much about their quality and user experience? For example, the power switch on my Nexus One (T-Mobile) was broken after only several months. If you search for "Nexus One Power Switch", you'll see mine is not an individual case but a very common defect caused by serious design flaw. Its screen is too sensitive, I can never use it when lying down on bed.
Google is a company that has virtually no any kind of end user support. When you buy a Google product, what you can expect from them? Even if Google really wants to take care of these phones, how can? Google does not make these products. Soon after the release of Nexus One, for example, Google had to hand over the support task all back to HTC.
I, like many perfectionists, used to like to have a Nexus phone because it is pure, no add-ons like Samsung Touchwiz, HTC Sense and Moto Blur. It turns out that the Tocuhwiz and Sense are actually much better than Google's user interface; they are not "add-ons" but must to have. Just for this reason alone, I would never want a "pure" Google phone again, especially for dummies like my wife. For example, she can never figure out how to turn the GPS on or off on her Nexus S 4G, because the stupid Google Power Control widget does not have any text label for the four confusing buttons.
Another reason for Nexus phones used to be that they were supposedly to get new updates sooner. Oh dear, endless updates, one after another, also soon turn out to be very annoying and not wanted. It is more a new problem maker than a fix. After the last update, for instance, the Nexus S 4G (Sprint) can no longer be charged fully to 100% and so far still no fix about this problem. And the worst part is: where and who we should report the problem to? Sprint, Samsung, or Google? If Google, where is the phone number or email address? You tell me.
Unlocked Nexus phones are very expensive and buggy without post-sale service. Their user interface sucks compared to Samsung and HTC and endless updates are annoying to say the least and they may add even more bugs other than offering fixes. Why you want to pay more to get such phones? If you need a unlocked GSM phone, you can buy an AT&T phone from eBay and Amazon and then buy a unlock code on eBay for few dollars to easily unlock it.