If you have it enabled it will fall from H to 3G in 7 seconds when connection is idle, otherwise in 30 seconds. 7 seconds are to short it will lose time and battery renegotiating to H to often. Ex: when you read your rss feed it will renegtiate higher speed for every article you read, very annoying.
Belive me, you are better off by disabling it.
History: Orange SPV c500, Qtek S200, HTC P3600, HTC Diamond, HTC TouchPro, HTC TouchPro 2, HTC HD2, Samsung Galaxy S, HTC DHD, Samsung Galaxy S2, Samsung Galaxy Note, Asus TF201, Samsung Galaxy Note 10.1, Samsung Galaxy Note 2, Samsung Galaxy N9005].