I am a hobbyist machinist. I bought a used Darex E-85 (like the E-90) fairly cheaply a couple years ago after I came into possession of a big pile of dulled or grossly abused end mills that came with a small vertical mill I bought. I hated to throw them away and was interested in learning about sharpening end mills anyway--a challenge.
The Darex is certainly better than nothing but no professional would mistake it for a serious precision machine. A certain amount of user dexterity, feel, and practice is required. If you are using CNC machines, forget it.
Realize you need an air compressor to run it. Mine is a 5 HP and it is just enough. My air spindle is a bit abused, thus requiring more air.
I feel there is one design problem. The air spindle uses a collar clamped to it which locates the flutes during end sharpening. Problem is, the collar is clamped with just a set screw. The set screw will distort the shaft enough to cause binding of the air spindle when it is used to sharpen the flutes on the side of the mill. That results in some uneven grinds in places on some flutes.
Have also noted some run-out problems with some end mills I have sharpened. Seem to get a high flute sometimes. Do not know why at the moment.
On the positive side, owning it has allowed me to touch-up many dulled end mills that I would have had to toss or send out for resharpening. I also salvaged a number of really grossly abused end mills. That effort was not time effective, I just wanted to see what could be done. I have one Putnam 1/2 inch diameter end mill that I used for practice. Somebody had chipped it and burned it up by running way too fast and no coolant. The sucker was black, blue and all the edges rolled over. It looked as if the cutting edges had melted. Took a while but I returned that tortured piece of junk into a nice cutting end mill. I was real pleased with that.
Another advantage of the Darex is it can be stored on a shelf when not in use. Space is a problem in my shop and another free-standing machine on the floor was not going to cut it. I store it on a shelf and drag it out to the garage (where the air compressor lives) to use.
If you buy one, be sure to get the collet set and stylus with it.
Sharpening an end mill is not a quick job like sharpening a twist drill. A lot more surfaces/edges to deal with. Figure on 20 minutes plus, depending upon initial codition. The really trashed ones took 45+ minutes, but that was a learning experience.
Overall, I regard an end mill sharpener as a very marginal machine for a home shop--handy but hardly a necessity. The money would likely be better put toward a bigger lathe or such. Of course, it depends upon your needs. If all you ever mill is aluminum and brass, forget it. If you mill a good bit of steel, it is nice to have.
Am interested in seeing others opinions, too.