the brets are a neat army, but you'll have a bit of a learning curve with them in order to get them to really shine! (ie: the empire, dwarves & high elves have far easier learning curves!)
the main thing you'll need to learn is;
a) how to properly support your charge. you can't hit piecemal because your knights are not effective in subsiquient combat rounds... you need to inflict maximum damage with your charge!
b) how to protect your flanks. the lance formation, while insanely powerfull, is insanely weak defensively!
while your knights can hold out for a turn or so against opponents who have S4 or less, if they also get hit in the flank, you can say 'bye-bye'...
cheers!