I didn't switch from an ultimate 13 but did switch from a 13 w 4gb to an 11 ultimate.
It is slower, but it's mostly only noticeable if you're doing very CPU intensive work or are playing high def video that isn't played through the GPU.
What is very noticeable to me is how much more portable the 11 is. For me, I didn't need the extra power, extra disk space. The larger screen is nice, but for me the extra portability is even nicer.
Only complaint about the 13 is that it's much noisier when the fan spins up. Jet engine vs. nothing.
I did. Could not justify keeping an ultimate 13" and my macbook pro. The Air made the mbp redundant because it was really good. I went from the ultimate 13 to the ultimate 11" to the base 11" with 4gb. The last move was a matter of cost and a real hard look at what I actually needed.
I have no experience with this but based on comments on other threads and websites, 1.6 isn't much different than 1.86 GHz. Also, I will save like 200 bucks. I don't mind the smaller screen either or the loss of the SD Card slot.