MegaBloks are a great deal cheaper than Lego, and have some slightly different ranges, having a license to produce kits based upon The Transformers as well as being a lot more creative with their parts.
Sadly, MegaBloks parts are not as well manufactured as Lego, and are really not suitable for any advanced modelling. Their tolerances are much lower, and when stacked, look rather like a badly built wall. Small models and detail may be fine though, but they do not really have a range equivalent to Technic, and are absolutely out of the question for engineering.
Rumours are circulating that the StarTrek license has gone to MegaBloks.