I had Canon 50 mm f 2.5 Macro lens (that I used with Canon Digital Rebel and later, Canon 5D), and I upgraded to Canon 100 mm f 2.8 Macro.
The 50 mm one was very sharp. Unfortunately it does not have ultrasonic (focusing motor), so it focused loudly. The lens barrel moves in and out when it focuses. But again, it was (I should say, "is" very sharp. Flat objects, i.e. a map, remained totally flat), even when mounted on a full frame Canon 5D--it worked fine. I needed more distance (working distance from the lens to the object), so I got the 100 mm Macro (and gave the old 50 mm to a friend!) -- and I like it even more. It focuses without a sound (due to ultrasonic motor) and the lens does not extend in size at all (internal focusing). Furthermore, it has (that 50 mm macro does not have) FTM (full time manual focusing), which means you can touch up the focus at any time, even when you're in AUTO focus mode. The focusing ring is super smooth and you can turn it beyond both ends. The focus ring does ** not ** turn when it focuses by the way (which is a good thing). Lastly, it focuses up to 1:1 (life size).
Hope this helps. :-)
p.s. It will work as a 165 mm lens on your cropped sensor XTi. A little "long" for a headshot. Try it out at a store, you might like it. Unfortunately it costs almost as much as XTi body! Get a filter and a hood if you end up buying one.
p.s. # 2. Talking about a "L" lens--this 100 mm Macro is no "L" (I have a few "L" lenses :-) but it sure feels like one. I am glad they don't call it an "L" lens--it might give them reasons to sell it for more.
# 3 Forgot to mention that since 50 mm Macro does not have FTM, you can't touch up the focusing while it is in AUTO focus mode--so you have to switch it to M (from A)--and when it's in A (auto focus), the focusing ring turns when the lens is focusing. A bit annoying, but I got used to it. Once I switched to 100 mm Macro, I couldn't go back.. but again, it's a really sharp lens.