I use an EF-20 tethered to the X100 using a Canon OC-E3 TTL cable. This gives me the freedom to have a more three-dimensional illumination. Using the Flash in TTL mode while setting the camera to slightly underexpose ambient light can yield some very natural pictures that wouldn't be possible w/o flash.
I'll post example pictures when I'll have processed my last batch of party pictures.
PS: If I need fill, I mostly use the built-in flash. The integrated flash hardly produces any shadows as it is so close to the camera's optical axis. That's a good thing for fill-flash, but very annoying for exclusively flash-lit images.
FujiFilm X100, Fuji X-Pro 1, Konica Hexar RF, Hexanon & CV glass & Nikon Coolscan V ... plus a big, bad DSLR
My RFF Gallery
, My Flickr
, My Ipernity
, all presenting different bodies of work