I already replied to a PM from MortiisMachine, but thought I'd post up a copy here...
I usually wear a U.S. 9.5 which is a UK 9 or a EUR 43. That is what I tried on in the store while over in Germany last fall and they fit absolutely perfect. I'll say that I have a very normal feet in shape and size. Even in ski boots, where everyone else needs their boots blown out in one spot or another, mine always fit perfect. Unless one has an exceptionally tall instep or a very wide foot or extra large calf, I don't see how the boots could fit tighter than the same size in any other boot or shoe. Obviously trying on is best with the thickness of sock you'll normally wear when riding, but I couldn't be happier with the fit of mine. I've only managed 5 or 6 rides with mine since I bought them as winter hit, but since they are leather, I imagine that, if they are a bit snug, they might stretch a small amount as they break in like all other leather boots I've owned. but since these fit just right, I don't think my foot will make them stretch much at all. As a side note, this is my first pair of Sidi, so can't compare them to how past Sidi boots run. All I know is I can't remember the last time I bought a shoe or boot size other than 9.5 US and these 9.5 fit me perfect.
That's the long way of saying I found them very size true unless you have exceptionally wide feet or a tall instep.