I think that if you gave half your class Nspires and half TI-83/84 and did not help them at all, the ones with the Nspires would find things faster.  The biggest problem I have is that I am familiar with the 83/84 and it's hard to find the time to learn something new.  Even after taking 2 workshops, I am still finding some things difficult, but with a little more time, I think/hope it will be worth it. 

One positive note - I have my kids explore the calc to discover the key sequences, etc.  They love finding a shorter way than me (or sometimes finding out their calc can do something I didn't know) and if they find it on their own, they are more likely to be able to find it again.