Safale 04 is my favourite yeast. Does the job fast, drops to the bottom quickly and leaves enough sweetness and flavour to make it all worthwhile, especially with flavoursome malts like in a porter/stout. Is an almost idiot proof yeast to my mind, and I'm an idiot believe me. Also makes bottling a dream as it sticks like sh!te to a blanket to the bottom of the bottle.
I was brewing a barleywine and I split the 5 gallon batch into two carboy during fermentation. In the first one I used SafbrewS33 and in the other one I used Safale S-04. After one week, I drank a sample (and took a reading) and I prefered the Safale one. Unfortunately, I mixed both into one carboy.
It's a great yeast for "sweeter" ales. It's not a great attenuator, but that's OK in a lot of styles. Be careful not to let the fermentation temps drop, this will cause US-04 to go to sleep (i.e. stuck fermentation). It is vastly superior to Windsor, which is targeted at the same styles.