I ordered an Alfa APA-M04 directional panel antenna to replace the almost- broken original antenna on my Buffalo WHS-HP-G54 router. $9 on Ebay.
As compared to the original Buffalo antenna in the horizonal position, the Alfa antenna is either the same or worse at any testing point. Also, I don't really see any front-to-back directionality with the Alfa even though the docs say there should be some. In other words, I got the same results whether the antenna was pointed forward or backward.
So, at any point along the direction of testing, the Buffalo horizontal antenna provided a stronger signal from my router as shown by WiFi Analyzer on my Android tablet.
It's puzzling that there are a lot of positive reviews on this. So far as I can tell, it's basically no better than a normal antenna.
Well, you win some, you lose some, and some are rained out.