I have trofast, they are labelled (yes seriously) ds1 is almost 6 and can read, ds2 is almost 3 but knows where stuff goes.
I have the small, medium and large tubs, they are multi-coloured.
The large ones are for "transport" so anything car/plane/digger etc etc, "dinosaurs" etc
The medium ones hold train track, musical instruments, jigsaws without boxes (destroyed by ds2 )
The small ones are for paper, pens, glue (high up and out of reach of ds2)
My playroom looks like Anthea Turner organised it but it works. I am very anal organised as I want to be able to find everything without searching the entire house.
I too store the large stuff like garages on the top because it isn't very high so they use it like a table top.
I did have the argos ones but all the boxes are the same size and I have had another storage system from GLTC but that didn't work so well. Trofast is fantastic.
Maybe you could have a day trip to ikea mine is 5 miles away is there an emoticon for spending?