I personally haven't used the extender kit but I have received very positive reviews about it. If you are really concerned with the battery or are a very heavy user, you can double your battery backup if you buy the extender kit.
Just carry both batteries with you, once one runs out you can put the other one in. And charge both overnight or perhaps even have a desktop charger to charge one while the other one depletes.
Keep note that if you use the extender kit backplate (which you would have to), the original battery being thinner will not fit tightly. So when you put the original battery in just put a piece of of folded paper between the battery and the backplate for a tight fit.

 

I got it a few weeks after getting my GS2 and while it won't radically change your experience, it will eliminate any worry of running out of juice on a day-to-day basis. I get about 40% more battery life, which means that except for extremely heavy use, I ALWAYS get to the end of the day with extra charge to spare, and I keep the original battery in the car in case I need to do some unexpectedly long GPS runs . Light users should be able to keep their GS2 away from the charger for at LEAST two days.
The only downside is that most hard cases will become incompatible. You will have to go for the soft-ish ones, like incipio.
Whatever you do, don't get the cheap, third party batteries. When it gets to batteries, you get what you pay for.

 

I cannot agree with this more. In terms of batteries and chargers ALWAYS go for original accessories.

 

It should be a worthwhile investment go for a desktop charger too if it's in the budget.