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Cakewalk's Reason-based emulation of the legendary LA-2A compressor (itself a version of its CA-2A VST) is a very versatile tool for getting live recorded elements to either pop or sit correctly in a mix, or for taming some wilder electronic sounds.
I have personally found that this compressor is excellent for vocals and guitars, and I've had a fair amount of luck applying it to live recorded drum tracks, as well. The presets are useful, if only as a starting point, but it's essential to learn to tweak the RE-2A by ear to achieve the best results.
A word of caution: this is not a very transparent-sounding compression tool. With that said, I really love the warmth it can add when used properly. It gives vocals a certain live quality that additional processing sometimes seems to take out of even a well-recorded signal. I don't apply it to glossy-sounding synths very often for this reason; using the RE-2A may undesirable coloration of certain instruments that are best presented with a pristine, digital sheen.
In summation, I would highly recommend this studio effect for all Reason users, but especially those who mix live recorded tracks in with their programmed electronic elements.