iZotope Ozone 5 by spiderman
After a few months of use, I'll say that Ozone is definitely worth the money. The maximizer and equalizer alone are worth the price. The stereo imaging tool is also very useful. I find the reverb to be a nice touch at times but nothing special in greater landscape of reverb processing. Ozone has SIGNIFICANT latency when used within multitrack projects.
For the DIY home-studio type the plugin is fantastic. It's a swiss army knife for certain. The presets can be useful for learning more about mastering approaches or mastering theory but aren't recommended, since mastering should be tailored to each track. For professionals Ozone is useful but not a first choice for mastering.