As with most of the plug-ins Variety of Sound offers, Density it at once both highly usable and (somewhat) frustrating. That does not mean it isn't great; it is truly awesome. Let me explain:
From the developer's blog to various user comments and review sites, one common theme about this plug-in stuck out in my mind: "subtle." And that's honestly the best way to put it when describing how and when to use it. It's ultimately best used in multiple busses/tracks than on just the master alone (though I do have a tendency to do that when mastering as well). It's CPU load is probably the smallest out of my entire VST collection, so it never becomes bothersome or questionable to try and throw another instance in the DAW. The seemingly slight usage of several instances of this VST on a mix shows up once it all comes together, it's just the "getting there" part that's difficult.
Which brings up the frustrating part. As much as I know (or think I might know) about mixing do's and dont's, there's always a reflex in me to turn everything up too far until I notice a clear change in the sound. This ultimately ruins everything; though if that's what you want to do in the first place, other plug-ins clearly have that advantage over Density. It takes a good deal of discipline to make VOS plug-ins, and especially this one, work for you. However, it is possible and worth the trial and error.
Density MKII is a nice sounding compressor/limiter to put across the bus. While it doesn't work for everything, it does work well for some mixes and drum busses, adding a nice glue to pull everything together, but go easy, as it is easy to add too much of a good thing. I want to give this plugin a 10, because it does sound good, it has a lot of control, and it is laid out well like the other VOS plugins. The only place that I think it falls short is in the timing parameter. As good as this plugin sounds, I find myself wanting more control over the attack and release times, which would give it the needed versatility for using it on more projects. I'm sure that Bootsy had sound reasons for designing the timing parameter in the way that he did, and I respect his creations for what they are. A lot can be said for plugins that both sound good and are easy to use. Also, Variety Of Sound just released a new compressor called ThrillseekerLA, which should cover other duties for which Density MKII isn't suited for. I haven't had the chance to spend much time with Thrillseeker yet, but I am looking forward to it. Maybe it can also cover some bus duties, picking up where Density MKII leaves off.
To sum up, Density MKII is a great plugin when it fits the situation. Be sure and check out VOS's other great plugins.
I've been searchin for good (and cheap) character compressor recently. For that, one must demo a lot of stuff. :D Density didn't fall into that category that much, but it sounds amazing even compared to stuff I demoed and I'm kind of sad it has no reviews. :)
It's not a pure model. Demoing OldTimer you get much more of character. But Density makes it's own magic in it's own way. Sometime, especially where you have lot's of creative distortion, I can imagine, character compression could oversaturate your material. But what if you want it pop up a bit anyway? That's exactly what's Density amazing for. It really feels like some combination of analog model and DSP.
Density brings interesting "clearifying colour" to your material. It's not saturation, it ... "something" and it sounds great.
Can't it explain really as I'm a bit newbie to "out of DAW compressor" compression. :D But it's free so nothing stops you from trying yourself. PS: If you need some saturation anyway, look at Tesla, another magic plugin by this developer. :)