The only time I go to 2.8 is when I don't have a tripod and I need to take the shot. The area in focus using 2.8 and at 1 to 1 is very very low, so without a tripod, you'll often move out of the DOF range just by pushing the shutter button. I use the lens on the 20D which has a 1.6 crop factor, so I need to have a shutter speed of at least 1/200th to feel safe when shooting without a tripod.
Here's a tripod shot of a quarter laying on top of a penny. I focused on the word "Liberty" on the quarter. I shot it at 2.8 at 1/100th. I set it up so that I could get as close to the coins as I could for 1 to 1 so the front of the lens was 6-7 inches from the coins. Of course its really 1.6 to 1.
I think it will give you an idea of the small DOF. As for all lens, if you stop down a bit you'll get some extra sharpness. But this one came out pretty well.