Basically, the SP1 setup process is an agonizingly long affair, at least using the interactive standalone install process depicted here. (So-called express installs should be similar, albeit with some online component downloading.) I will examine slipstreaming in the future.
In any event, an interactive install requires several reboots--five, if I counted correctly--and a surprising amount of time. When all is said and done, what you're left with a system that's apparently visually identical to the one you started with. It's too early in the SP1 development cycle to worry about such things as performance and reliability, but so far so good.