It's better to resurrect an old thread that already discusses this topic than it is to create a whole new one. XDA rule #1: "search before posting". If a thread already exists that deals with what you want, DO NOT create a new one.
And to the recent poster:
I'm not sure anyone has done "scientific" testing to see whether or not "fast dormancy" helps. Based on the available generic GSM literature on the matter, it would seem like a good idea. Faster data call spin-ups, and less battery use. However, AT&T notoriously doesn't follow the GSM spec fully, and often times picks and chooses what to deploy on their network.
I suppose the best way to test is to run a whole bunch of speed tests with and without FD turned on. And to live a week with it on, then a week with it off, and compare average battery life. And if you can get another 15-20 people to do the same, then we do some statistical analysis on the results, you might actually find out whether to enable it or not.
Short of that, the rest is heresay and wildly subjective opinions. Any given individual's usage patterns may vary so wildly, that without looking on aggregate (and with correct analysis), it's almost impossible to tell whether or not specific "insert-preference-here" actually has a positive effect or not. It is worth mentioning that fast dormancy does appear to be disabled by default on AT&T based ROMs for the GS2. Of course, such a setting might have been over-looked, or possibly even disabled despite potentially helping performance/battery on our phones. Who knows. Unless you have a friend who works as a network engineer at AT&T, we might ultimately never get an answer on this... :-/