Summing up from my comments above:
How do signal boosters work?
They work by forcing the device to do a re-connect. Though the device is permanently watching its neighborhood (for the case the signal gets too weak), it does not switch for just a trifle. Instead, it tries to weight the benefits against the costs: switching to a different cell has some signalling overhead to be considered. So if it's just for a trifle, it wouldn't switch. But if the other signal source is twice as strong, for example, it would. And it certainly does switch once you lose signal, even if there's just a weak signal available to switch to.
If it switches, it always choses the best and strongest signal available. And as I said, when you lose your signal it does switch: that's exactly the concept of the booster. By going to airplane mode, it makes you lose the signal, so switching on the radios makes the device connecting to the strongest signal. If your lucky, you see one more bar. If there was just one cell available, you just wasted juice.
IMHO it's not really worth it, but everybody needs to decide that himself/herself. Improvements are rather marginal/cosmetical in most cases, if at all.
Ways to boost without a booster
Well, if you followed my above writing, you might already have guessed it: Long-press your power button, and select "airplane", then do it again. Or, open your notification bar and tap the plane icon, and repeat it. Done. The difference is you need to do it manually – while the app at least partially automates it (maximum one tap with the app, while 4 manually).
There are apps available which track cell signals. Some do even track the neighboring cells (most of them only work with selected devices). If you find one that does work with neigboring cells and your device, you should be able to tell the real numbers – and see if it would be worth to "boost", or if it rather would be to "boast" :).