According to these tests, latex tubes cut rolling resistance anywhere from about 1.5 to 3.3 watts per wheel, depending on the tyre. That's quite a bit, when you consider a pair of Zipp 404s cut not much more than that in aero drag, assuming the figures cross over. Many of the same clinchers are tested with both butyl and latex tubes.
http://www.biketechreview.com/tires/AFM_tire_crr.htm
I've been meaning to get some for ages, but I keep forgetting. Not many shops around here have them, and they're a bit expensive. They're also meant to be harder to repair, but I dunno if that's a myth or not.
'Some' say that latex tubes is a big part of why tubulars are so nice to ride.