Megacorps themselves are very meh, but the other goodies such as the extra megaconstructs are awesome.

The corporations themselves are super weak though, since there's a lot of restrictions on where you can set up shop. If you don't have anyone you can trade with nearby then you're just a drastically weaker empire just like any other. Criminal organizations are a cool concept, but it's completely wasted because the AI spams enforcers like crazy and players will just shut you down instantly. Everything about the corporations needs some desperate patchwork, and Paradox doesn't tend to fix old DLCs. They make okay AI players, but don't buy it if you're expecting to have fun being a space Walmart.