Skype just released a native client for iPhone earlier this week and I downloaded it as soon as I saw the announcement.
I have been using Nimbuzz for Skype chatting for a little while but it has pretty high latency. I did a quick, non-scientific measurement and the latency can be as high as 2-3 seconds sometimes.
The iPhone Skype, on the other hand, have very low latency and better voice quality. I was a little bit disappointed to find out that I cannot make voice calls over 3G network. However, with the crappy slow Vodafone 3G network speed and monthly 250MB data cap, I don’t think I have missed out too much.