I use akAuto3G which is similar. AkAuto3G automatically turns off 3G and turns on Edge whenever you're connected to wifi or when your phone is locked. It works as advertised and the dev is extremely responsive. I pointed out 2 things to him and he's implementing both. As of right now, every time you lock your device, even if its only for a quick second so you can do something else with your hands, and then immediately go back to using the phone, as soon as you unlock, the phone will turn off edge and turn on 3G. This can be annoying when you only locked the phone for literally a few seconds and now you have to wait for the radios to change. So the dev is implementing a time delay feature to avoid this problem. That's a very welcomed addition. It's in the next update too which should be live soon. The other quirk I learned was occasional dropped calls. Before i purchased the tweak I emailed the dev and asked about dropped calls when going from connected to wifi at home to being outside and no longer connected. I asked because obviously once you're not on wifi but youre using the phone, it will turn off edge and turn on 3G. It won't go back to edge until you lock your device. He told me that sometimes the call will be dropped and other times its fine. Regardless, he is working on a permanent fix to force the tweak to wait until the phone call is officially over before switching the radios. That was my biggest concern but because he has responded to all of my emails and was honest with me, I purchased the tweak. Luckily I haven't experienced any dropped calls yet and hopefully he'll figure out a way to never let that happen.
Either way, once he adds these two updates ill be extremely happy with my purchase. Right now I'm already happy. I've been having some of my best battery life with my 4S on 6.1.2. My battery is significantly better now than what it was when I was on 5.1.1, 6.1, and 6.1.1. For me, 6.1.2 blows those firmwares out of the water in terms of battery drainage and I'm hoping this tweak helps squeeze out a little more juice! So far it's been great. I'll keep you posted if you want.
Edit: I am with AT&T and I live in a city that has terrific 3G and Edge networks. No matter which radio I'm on, I always have max reception. Although now that I'm using this tweak, I am never on Edge when I'm actually using my phone. Before when I'd rely solely on NCSettings to toggle my 3G radio on and off, sometimes if forget to turn 3G back on when I'm quickly checking something on safari or an email. Now whenever I unlock my phone, this tweak automatically turns on my 3G and I get the speed boost and then once I'm done and I lock my device, it automatically enables edge to save battery! And I still get all my notifications because I keep data turned on at all times. Needless to say, as a person who still turns on edge to squeeze out a little bit more battery life, I'm happy I bought this tweak. (I decided to type all this because I have found little information on this tweak's actual real world usage. Now at least I put a little bit of information out there for anyone who is curious.)