I bought a pair of these when they where released, i have been using them for 5 months.
I find them more useful for mixing than tracking. It's a compressor that offers a lot of possibilities but it takes time to learn and get "right".
Between the Fet, the Opto and the crossover, you have a lot of interaction and possibilities and i find it too much to deal with when trying to get good sounds fast.
On one hand it's great because it gives you so many different great sounding options, on the other hand it becomes counter productive when tracking, unless you are verry confident in your on the spot decisions.
I end up favouring other options like Distressors, RND 535, UA 1176, Api 527, which i find easier to get a good sounds from quickly, and easier to get "safe" tracking settings from.
For post processing things though, i love the OptoFet, my pair are mostly used as main vocal processing and as bass guitar processing, they live in the insert of a Neve 1073 DPX.
On the voice the classic "fet first, opto second" works a treat and combined with the Neve EQ it really allows you get the perfect vocal track, it just sounds right.
On the bass, the ability to play with the crossover and to switch fet-opto or opto-fet really allows you to position the bass in the mix in many different ways, i like to use the fet on the low end compressing one or two db with a fast attack, and then the opto on the higher frequencies with a much slower attack and more compression, You can then use the crossover frequency to really tune the tightness of the low end, it works amazingly well.
It's really fun to experiment with, but it takes time, it's a compressor you sit in front of for a while, trying different things, and then suddenly you get what you want and it sounds better than any other hardware you have or any combination of plugins.
I don't really use the compressors for anything else, the 1073 DPX + OptoFet is my main vocal and bass guitar mixing chain, i love it, i see no reason to change it.
