The standalone IDE will be useful for people getting started with Clojure, since it'll provide a more integrated experience - we bundle lein so that Windows developers don't need to download and install it to get going, for example (although this is currently not exposed to the user, coming soon). Clojure is great but the getting started experience is still not great for total newbies. I hope to provide a great environment that'll be basically a one-click download to get started.
It'll be similar to PyCharm in that the standalone IDE will be basically a cut-down Community edition plus the plugin, right, although it can be customised more heavily for Clojure if required. The licensing will be the same for the plugin and the IDE (i.e. your licence will work for both), so if you already own IntelliJ Ultimate it would make sense to stay with that. Unfortunately I don't have the luxury of offering the plugin free to IntelliJ Ultimate users as JetBrains can do with PyCharm.