Firstly, the weaknesses. As a Best Of, it's never going to flow as well as the albums; indeed, MA albums often flow better than many so-called concept albums, so the sequencing here would be jarring regardless. Secondly, it doesn't suit those who don't like Massive Attack, or their music, although whether you can blame the album for this is debatable. I can respect the opinions of such people; but they are wrong.
Strengths: mainly, this doesn't fall into the traps of so many other compilations by other bands, a particular one being the tendency to lift half of an album critically hailed as a "classic" and then to bolt on a couple of songs from others, e.g. like Blur's Best Of which lifts six songs (out of 18) from Parklife and then use no more from three from any other album. Collected is roughly even between all four albums (although Mezzanine gets an extra cut- a slight at departed member Mushroom perhaps?) and the DualDisc features videos from every single from 1990 onwards.
Overall, it's an excellent introduction; I only had one album before buying this, and it convinced me into getting the other three. In that sense, it does its job.