There is virtually no lag on Nexus S. I don't know what I have to do to make it lag, it's simply impossible.
On the other hand, my Desire was laggy at times, and it would crash on app open numerous times.
As you can see, I had HTC Desire before Nexus S. I sold my Desire with one intention only - to buy Google (Samsung) Nexus S. Like many others already mentioned, there is bunch of reasons to make a switch. Maybe it's just me, but for some reason I started to dislike HTC's design. All those phones look kind of outdated, every single model has that ugly-ass "chin" at the bottom of the phone. But all that to a side, more important things are of course performance. 1GHz Hummingbird vs 1GHz Snapdragon, no brainer... can't even compare those two. The quality of the sAMOLED screen is just amazing, blacks are truly black, colours as vibrant as they can be... front and back camera on Nexus S and I believe bunch of other stuff that works in Nexus S favour.
I have to say, I have no clue why all that fuss about SenseUI anyway. I liked it, but it's not like I can not live without it. Pinch to access all screens, big deal. You can do almost the same thing with Nexus S. Just hold your finger for few seconds on MENU icon (grid), or any side with those little dots representing all your screens, and you'll get 5 preview thumbnails where you can easily jump back and forth through all your screens. Stock Android OS is quick and it does it's job perfectly. Not to mention updates coming directly from Google, meaning less time to wait for some third party developers team to develop for that particular platform.
So far, I am impressed with overall performance of Nexus S, and I am super happy with this device. I loved my Desire too, but nothing like this.
Also, not to forget - all those Desire problems with MultiTouch:
http://www.youtube.com/watch?v=hVlsRCMltDg
Here is one quick video preview how MultiTouch works on Nexus S (5 registration points is no problem for Nexus S):
Google Nexus S (Samsung) true MultiTouch capability on Vimeo
@Anthony1 - Hope some of this helps in making your decision
