Just did the first proper SIP phone call from the Galaxy S to test out the quality and data usage.
The call quality was pretty decent on my end. I could hear the other side well in duplex mode, there was no echo, and the whole experience was quite pleasant. There were initial quirks on the very beginning after the connection has been established but that went away after about ten seconds or so.
For the other side, the quality of connection was not as good as it was for me. They could hear echo and their own voice. This really affects the conversation, at least for me. I can't stand hearing myself repeating what I just said. However, in this case the conversation went pretty smoothly.
So, CSipSimple justified the praises other people had for it. The data usage for a 13:04-minute conversation was somewhere around 6.5 MB. It is hard to say because I did not note down the actual amount of data used before the call. It was somewhere in the range of 15 MB. And, at the end of the conversation the usage was at 21.55MB. That is pretty decent usage amount. As in the data usage is negligent - about 0.5MB per minute of duplex conversation.