This is probably my favourite lens both to use and the results from it. Being released just at the time we wanted to get a full macro lens we got one very soon after it was released, but at a good price. It's seen steady use ever since.
Being an L series lens it is expected to have good build quality, excellent optical quality and some level of weather sealing. This lens ticks all of the boxes as far as I'm concerned and we have had no problems with it. It comes with a lens hood (really, all lenses should come with these, stop being stingy Canon!) and a lens pouch. The pouch is fine to prevent knocks and scratches but you'll probably want something more sturdy if traveling a lot.
Although quite pricey compared to the non-L 100mm macro (optically there's very little difference) the thing that really makes this such a useful lens is the latest generation image stabilisation that has been added. It is quiet and very effective, both for normal use and macro distances. This makes the lens much easier to use for hand-held macro work, something very handy when chasing skittish insects.
With excellent sharpness and a reasonably fast aperture it can double as a portrait lens. It's a little too tight on a 1.6x crop camera (160mm is more telephoto than portrait) but on a 1.3x camera or where you have plenty of room it can take lovely portraits. It also serves well for some action shots eg photographing certain green and red heavy goods vehicles on a motorway from a moving car (don't ask). The focusing speed and accuracy are surprisingly good in most situations for a macro lens.
Mostly, though, it's as a macro lens where it sees most use and it doesn't disappoint. With the combination of sharpness, excellent bokeh and image stabilisation we've been getting consistently pleasing results, only limited by the camera and operator skill rather than the lens.
For more magnification we often add on extension tubes (usually 20mm to get closer without having crazily shallow focusing distances) but 1:1 is enough for most of what we want.
The only downsides to the lens are the lack of a tripod foot (we got a 3rd party one for the odd occasion we need one, but the official one is difficult to get and ridiculously expensive) and 100mm having a little too short a working distance for more nervous critters. 150-180mm would probably be easier to use with some insects but then it would lose most of its value as a general purpose and portrait lens.
While it was until recently our most expensive lens it probably has the best fun/£ rating of all of the lenses. I don't expect to replace or upgrade this lens for many years to come and would happily recommend it to anyone that wants a macro lens and can afford it.