First and foremost, if you don’t know what a de-compressor and a de-expander is, don’t be ashamed. I had never even heard the terms before I tried Flux Solera. They work basically the opposite of a compressor and an expander (and here most people in the world thought the compressor and the expander was each others opposites!).
So Alchemist is not only a multiband compressor since it can let you use plenty of other dynamic tools as well. I think this concept is truly interesting. Think of using a compressor on 400 Hz and an expander on 800 Hz with one single piece of equipment.
Alchemist also has tons of Flux specific features such as Angel’s share and Hysteresis. Many of these features are very hard to explain, so if you’re interested in Alchemist I suggest you read about it on Flux’s website.
So is it any good? I haven’t tried it yet, but speaking from experience with Flux other products (I have all of them) I would be surprised if it didn’t truly kick ass.
Alchemist is available in RTAS, VST and AU formats.