FireCobra is, quite simply, brilliant. Place it in the Master channel and it opens up your mixes so you can hear every part clearly. It brightens the whole thing and gives a professional "sheen" to a mix in a minute or two that you might otherwise spend hours trying to achieve. Adding it to a single channel is a little hit and miss - sometimes it makes a huge difference, sometimes it doesn't do much at all, depending on the source.
There are basically only 3 control knobs, plus a dry/wet balance, and you get a dozen or so presets. But there is a lot going on under the uber-cool interface. Try a preset or just tweak it from the INIT patch yourself. Either way, you'll get an amazing result almost instantly. There are a couple of selector switches but you only need to use those if you are having problems. I've been using it for a couple of months now and haven't had to bother. You also get input and output gain sliders so you can gain match to the untreated signal, just to make sure it's actually better, not just louder. The three main controls are INTENSIFY, which does most of the magic via some machine intelligence algorithm. The other two are SMACK and ANALOGIZE, which both add extra harmonic content to fatten up the sound. Analogize is a lot more subtle than Smack, so I tend to use it on mixes, where I use Smack a lot more on synths and drum parts.
I find that the better the mix, the less pronounced FireCobra's effect is, which is cool because you can throw it on a problematic mix and it might solve your problems for you. It certainly saves me from spending too long polishing up mixes for our live sets. I just get it close and let FireCobra do it's magic. And sometimes it really does feel like magic.
Yes, it's expensive and, yes, I got it when it was on sale but, honestly, if someone showed it to me and I heard exactly what it's capable of, I think I would be happy to pay full price. It is easily the best plugin I have ever put on my master channel and what it does is absolutely worth what they are charging.