I just bought Marquis Compressor after trying out the demo version last night (same as the full version but with periodic beeps). I compared it with Kjaerhus Gold Compressor and some freeware ones I had around.
The main attraction for me was the clever metering system which simultaneously shows peak and several average levels to give a visual picture of the input, output, and compression levels. Other visual cues are the knee-shape display and program-dependent release contour.
But what surprised me was the sound! With the same threshold/ratio/attack/release setting and the same knee shape, Marquis sounded much better on a vocal track than the Kjaerhus compressor. The latter emphasized sibilance and "F" sounds in an intrusive way. In fairness, this was the only comparison I made, and perhaps there are other parameters in the Kjaerhus compressor to deal with this.
Last but not least, check out the "Key Filtering" feature. This again is a great visual interface to a side-chain-like capability. For example you can get a very good de-ess effect by dragging up the EQ band around 8 kHz, so that this band has a greater weight when triggering compression.