Proximity is an extremely valuable tool for mixing, and not only music, but speech, theatre, FX and everything where you need a bit of hand-crafted realistic impact.
Pros: • carefully imitates an object getting closer or farther, including absorbing lows and highs, narrowing stereo field and so on • perfect for fade-ins, whooshes and reversals, as it does much more than just a volume • invaluable for mixing background music and noises, for very natural fade-outs • you can select different algos or even turn them off one by one, tweaking the combo to fit your sound best • easily cuts a "digital sparkle" off a nasty synth or brings the glory of your stereo bongos right upfront.
Cons: • the author is not praised around for it on every corner.
Once you get to know it and understand what exactly the effect is, this one is superuseful. Writing a music with it is fun, but it certainly shows its full value when one's mixing radio dramas or such — without this tool, the workflow could be so much more clumsy and problematic.
A top notch thing. A must have for everyone related to sound production.