Agree that immigration at IAH is well organised and works well. Queues are indeed dependent on which other international flights arrived just before yours but in my experience they've never been too bad. The fingerprints / twenty questions routine is of course a pain in the @ss but that's not unique to IAH.
The BA lounge is usually pretty crowded though and a bit dark and gloomy. The First lounge is small but has better daylight. (I had the good fortune to get a free upgrade from Club in my gold card days).
Baggage reclaim can be a bit slow but all in all IAH is a nice airport to travel through.
Been a while since I was at DFW but I think I would choose IAH in preference (although the Houston/Galveston area is in a bit of a mess at the moment, courtesy of Hurricane Ike)
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