MOST aluminum OEM production heads have cast iron guides. I see very few worn guides, and the heads I see usually have 125,000 + miles on them.
My experience with bronze guides is extensive. I personally used bronze guides for most of my guide replacement between 1985 and 1995 or so.
I found that they work best with a chrome-plated valve stem. An unchromed stem will wear rapidly both the valve and the guide.
Bronze guides CAN be sized tighter than cast iron, but there are some caveats involved. First, the valve seating equipment must be able to seat the valves to within the guide's tightness parameters. If your valve seats have .003-.004"" run-out, a guide sized to .0012" clearance won't work very well. Also, for a guide to be fit very tight, it must be straight and round. Typically, the best way to accomplish this is with a Sunnen valve guide hone.
The latest wrinkle in guide liner sizing is ball-broaching. It has supplanted sprialing and reaming as the more desirable way to size guides, in part because it provides a more intimate liner to parent-bore fit. Final sizing is still done with a hone.
Solid bronze guides (thick wall) are still sized by reaming and honing.
The bronze liners must be finished with a slight chamfer and with a honed finish. As was said by others, good installation techniques are needed for good service life from bronze (or any other) guides.
At this point I install more cast-iron guides than bronze liners. I feel that cast guides are more cost-effective and, for most applications, longer lived.
Most hot-rod type engines are not expected to last as long as a stock type of rebuild. I think that if hot-rod heads were to be checked after 60,000-80,000 miles, the mystique of the bronze guide may be a little dimmed.