SInce I started shooting with 1.6X crop sensor cameras in 2004, this has become my primary macro lens. Before that, on film (i.e., "full frame") cameras, I tended to use the 180/3.5 macro more.
With the cameras I'm using now, the 100/2.8 gives me the right combo of adequate working distance from shy or downright nasty, sharp-toothed subjects, but not so far that depth of field is ultra shallow so that I need to stop down a lot and use a slow shutter speed, to the point it's no longer practical to handhold.
The 100/2,8 also serves very well as a moderate telephoto lens for non-macro shooting, on any crop sensor SLR. The main reason this lens serves dual purposes so well is that it's got fast USM auto focus. If you are sure to use the limiter switch when shooting at non-macro distances, it's AF will not disappoint (the 180/3.5 does not focus as quickly at non-macro distances, so is less of a dual purpose lens, in my opinion).
Now, I know a lot of people use the 100/2.8 for a portrait lens, too. If anything, in my opinoin it might be too darned sharp for that purpose. You won't win points with your mother in law if you take her portrait with this lens and so precisely show every line and wrinkle! Still, it can be used for portraits, no doubt about it. Just soften her images a little in post production, or risk the wrath of your mother in law!
It also can optionally be fitted with a tripod mounting ring, very handy, but something that is not possible with some other macro lenses around this focal length.
One thing I've never been able to figure out is why the optional lens hood is so large. You certainly don't have to worry about stray light finding it's down there to the front lens element, when you use it! It's a bit of an overkill, but better too much hood than too little, I suppose.
Internal focus means this lens doesn't change length when focused, which is pretty amazing and convenient for a 1:1 macro lens. Earlier 1:1 macro lenses grew tremendously, with double heligoid designs. Some tripled in length, which moves the front elements awfully close to small subjects.
While I would agree that I.S. is nice to have on any lens and would be somewhat useful on this lens when using it as a moderate telephoto, I have to point out that I.S. is of little real use at macro focusing distances. There is one stabilized macro lens on the market now, and even it's manufacturer doesn't claim very much assistance from stabilixation with tiny subjects at higher magnifications. So, personally I don't see much sense in adding I.S. (and complexity and cost) to this already excellent lens.
The 100/2.8 Macro works extremely well with macro extension rings and the MT24EX Twinlite flash, too.