I think the Nexus S is a great phone, however:
1. Nexus S doesn't have an LED indicator light of any kind.
2. Nexus S CANNOT be made to work with Sprint Visual Voicemail. Period.
3. Nexus S doesn't have an FM radio.
The first point might not be a show-stopper for you, but I really like having an LED indicator for messages, missed calls, or however I configure it. Nexus S fans will tell you there is a "No LED" app in the Market that uses the lock screen to show different types of indications using icons for mail, VMs, etc. However it's a little sketchy to get it working the way you want (I played with it on a Nexus S in a Sprint Store).
The second point was a deal-breaker for me. I like being able to access voice messages visually in a way that lets me see which one is which, from who, by date, etc. You can't do that on the Nexus S...ever. It simply cannot be "provisioned" to integrate with the Sprint VM system.
The third point was semi-important in the gym, where a lot of the TV audio is broadcast over FM.
I had an HTC Hero before this, so everything on the Shift is nearly identical to use.
One thing about the Shift I DON'T like is the slight physical shakiness that comes with the slide-out keyboard design. I didn't really want a slider, because I'm fine with on-screen typing. However, this was the only phone that currently had everything I need, and isn't HUGE like the regular EVO or the Samsung phones.
Also, keep in mind the Shift does NOT have an NFC chip, so if phone-scan purchasing picks up, you'll have to add one in the NFC-equipped memory cards that may be coming out, or use one of the stick-on solutions or "photo of your credit card" apps that are in the works.
Also the Shift does NOT have a forward camera, so no video chat.
I wish the Sprint Motorola Photon were already here..or the rumored "HTC Evo 4G+" for Sprint...but neither will arrive for a while, and I was SOOOOO pissed at the "old" Hero lagginess that I was going crazy, so....Shift for now.