At work, I'm on a site where I regularly have 0 bars through the entire building, and sometimes a flash of 1 bar (rarely). Over the last 5 days that I've been there, I've had my Slot Cycle on different settings.... but let me take a step backward first.
The slot cycle change from 2>1 or 2>0 will also make your phone react more quickly when you push your Off/Wake/Sleep/End Call button at the end of a call. This eliminates a perceived 'Lag' that many people complain about, when it's just waiting to communicate with the tower (rather than the phone itself lagging).
Let me attest to the concept that when the Slot Cycle is 0, your battery gets OBLITERATED if you have 0 bars. Sitting asleep, my battery goes from draining 2-3%/hour with 3g on to 5-6%/hour with 3g on, when I'm in said 0 bar area. I can still make/receive calls (I believe our phone has a very strong signal amplifier - which is what is draining the battery when you're at 1/0 bars!) with 100% clarity.
The Slot Cycle at 2 makes that 5-6% (per hour on slot cycle 0) turn back into 2-3%, consistently. Over 3 non-talking hours I lost 8% (or 9% if I am bad at keeping time) with Slot Cycle 2, and 17% over a similar 3-hour period on Slot Cycle 0.
I've resolved to Slot Cycle 1 for now. Since my Verizon reception is great in most places, for the most part, the Slot Cycle Index at 0 doesn't seem to do much to impair battery life. But it seems, based on my involuntary experiment, that the change isn't so obvious (or much of a change at all) until you have BAD reception. When the tower signal is weak, your phone has to get 'Louder' to make good communication with the tower. And there goes your battery life (down the drain).