There's two grades of Madbulls: the "Precisions" available in .20, .25, .28 and .30, and "Match" available in .20 and .25.
MB Precisions are like other regular, quality BBs (KA, Bastard, etc): the .20 and .25 weights have large air bubbles in them, which aren't always centered (don't take my word for it, split open whatever BBs you have now and take a look for yourself). The .28s and .30s are denser and have no real bubbles to speak of (this is also true across all top tier brands).
The MB Match grades are slightly more expensive, but they contain much less bubbles and what few bubbles are there are very small and distributed evenly through the material. From my own experience I found the MB .25g Match BBs to be similarly accurate (indoors, without any wind/foliage to take into consideration) to .28s. Except cheaper and obviously less FPS loss. Obviously if playing indoors with lower FPSs, this is an advantage.
Here's a sample picture:
I split 10 random MB Match BBs (.25g) and 10 random BBs (also .25g) from another popular brand and picked out the best and worst sample from each. Madbull Match on the left, the other brand (lets say Madbull Precision, cuz they're basically all the same) on the right; best sample top row, worst sample bottom row. The image is of BBs split in two: I split them further after to see if other bubbles were present but those cross sections seem representative of the BB as a whole.