IMHO wifi is not a cure for crappy service. It only address's the data side of usage.
But that aside, for me(at&T - 9000), it has been pretty much toggle on, scan for networks, connect, enter network key(only if needed) and I'm on.
The speed depends on the wifi service, but it should be faster than typical mobile wireless speeds. Actual numbers are hard to gauge due to many factors like upstream pipe and how saturated the wifi router is.
It can and does save the wifi settings for later use.
I use it mostly at home and work.