As to the set itself, you get to make Optimus and Megatron as either robots or vehicles. Despite complaints from some about the lack of transforming ability on earlier sets, this hasn't changed and to convert from one to the other you have to take the model to bits. Personally, this has never bothered me. I like sitting and building Lego models and the children like smashing them up, so Kre-O is perfect for us.
Kre-O likes to mix up Transformer versions for its models and Battle for Energon is no exception. Optimus is a bit Transformers: Prime-like, but with a dose of G1 thrown in. Similarly, Megatron is sort of Prime, a bit G1 and some movie stuff thrown in.
The set is good in that you get to build both a goody and a baddy to fight against each other (and a little bank to fight over). None of the models will take particularly long to make, but they're quite fun to do - it's perfect for someone like my 6yo who likes playing with Lego but has a limited attention span. I've noted in an earlier review about Kre-O liking to pile up flat pieces and this continues, annoyingly, in this set. It's so hard to separate those pieces!
The single most annoying thing about Kre-O is that different pieces are required for each mode. Robot mode uses the most, but make the vehicles and you're left with a bunch of bits which are asking to be lost. Unless you do something like this...
I'm sure it wouldn't be too hard to ensure both modes used up all the pieces.
In addition to the models in the instruction book, you also get a few transparent purple pieces. If you buy all the wave 2 kits then you can put all the purples bits contained within together to build a big purple thing. It's possibly a gun. Unfortunately I'm almost certain that not all the sets were released in this country, at least I've never seen them all, which means completing this purple...thing will be difficult.
Overall this is a nice little set and at absolute bargain at current prices (I've seen it down to under £10).