I used to be dead set against anything but Lego in the construction toy department. I had a friend who wasn't as elitist, though. He bought some of the Mega Bloks Dragons line when it came out and showed them to me. I was impressed with the dragons despite myself, although I always hated the light up blocks. They went against every brick building bone in my body but I couldn't deny their supplemental playability...I guess the kind of thing a person might enjoy in action figures like G.I. Joe. I convinced myself to buy some after staring at them on the clearance shelf at wal-mart awhile. The sets were obviously inferior to Lego in virtually every way but their oddly shaped purpose pieces helped the constructions go together...easier? With less thought, planning, and brick-build time investment, certainly. I found that this took away from the whole Lego experience, dimming the creativity involved, but I can't really deny that once you get those bloks forced together they make fairly interesting set pieces. It would take me some time to purpose a Lego castle (which is, in my opinion, still going to be cooler looking due to the clean lines, clever construction techniques, the joy of a job well done, and the lingering feeling of happy you get from being able to play with toys) but I can build a similar Mega Bloks castle in less than a quarter of the time (which makes it more ideal for building a lot of terrain in a pickup game of BrikWars or something similar, god forbid). Recently, out of morbid curiosity, I picked up a small Halo set and, I have to say, I came to the same general conclusion. Mega Bloks has certainly stepped up their game. Maybe because they're getting real licenses? The bloks are devastatingly superior to what they used to be. They still don't begin to approach Lego quality, however, and probably never will. It does take much larger constructions for those little inconsistencies to build up and make further building difficult or impossible. If anything, in sharp contrast to their older and looser style bloks, my chief complaint about the set I bought is that the bloks stick too tightly. Which, again, is good for set pieces or display but not so much for creative building and re-building. As far as the figs are concerned, I'll say that I legitimately like the articulation found in the newer Bloks sets but they'll still never compare to the clean charm of a good minifigure for me, especially not with the Lego Minifigures line in place now.
Overall, I'll use them (or some of them) but only really in a supplementary role to Lego.