The Help files that come with Flex Builder claim that the HorizontalList control can have better performance than a HBox with a repeater:
"...performance of a HorizontalList control can be better than the combination of an HBox container and a Repeater object because the HorizontalList control only instantiates the objects that fit in its display area."
The truth is, this statement is false. The HorizontalList always instantiates one more control than is drawn in the display area. This extra component is created simply for measurement purposes and is kept in memory in case extra measurements need to be done. That's highly ironic, as I'll get to in a moment.
In a situation where the number of items does not exceed the area available, the HBox with a repeater component will instantiate exactly the number of renderers as items in your dataprovider, whereas the HorizontalList will instantiate one more. If your itemRenderers are heavy (for instance, if they themselves have a component that takes an itemRenderer), this difference can be significant. Additionally, Repeaters do offer the ability to recycle their renderers, which offsets most of the performance advantage you might get from a HorizontalList.
The second problem with the way the HorizontalList "does business" is more significant. If you have to use callLater to do anything that affects the size of the itemRenderers, the HorizontalList has already measured the component before the call that changes the size, and so the space it allocates to the renderer will not be the actual size of the renderer. And there doesn't appear to be any way to dispatch any events from the renderer or set any variables or call any methods that will convince the HorizontalList to use that extra, useless itemRenderer that is hanging around at the new and improved size to update the size.
Possibly you could subclass HorizontalList to make this work correctly, but why would you, since the HBox with an ItemRenderer just works?
Updated May 19: Today, I discovered that the HorizontalList doesn't always draw this extra renderer. If you specify a rowHeight, this step will be skipped.
Updated September 11, 2008: I think if I'd overridden measure() in my itemRenderer to explicitly report the size after the dataProvider was set in commitProperties, I could have gotten a HorizontalList to work. I later had a similar problem with a regular List, and overriding measure() fixed it.