A Google Books ngram result for “Lego bricks” and “LEGOs”.
The Lego Group themselves use the former; the latter is, as far as I know, exclusively known in the US:
@54_52_4f_4c_4c The correct one “I have a box full of LEGO bricks” … “LEGO”, because it’s our brand name, shouldn’t be pluralized. 🙂
— The LEGO Group (@LEGO_Group)//platform.twitter.com/widgets.js
Unfortunately, if the chart above is to be believed, the singular plural (as opposed to mass plural) form seems to be more popular now. I find the “Lego bricks” (or just “Lego”, for a bunch of pieces) to work far better. Ah well. Language, eh?