I actually now have three dado sets. I bought two of them directly, and the third came with a PM66 that I bought. Here is my opinions about them:
Forrest Dado King - This was the sharpest of the lot. Even though I was being careful when setting out all the baldes when I got it, I still managed to slice the crap outta my finger getting one of the chippers out of the "case". How does it work?? If I want a REALLY crisp flat-bottomed dado that is also smooth bottomed with little, or in most cases NO, chipping on the edges, this is my goto set.
Freud SD508 - This is my latest addition and came with the PM66. I have not done any side-by-side comparisons with the Forrest, but I think the Forrest is juuuuuuust a tad crisper. However, the Freud still leaves fine, flat-bottomed, assembly ready dadoes and in my opinion you can't go wrong with it. Now you might be noticing that I am comparing a second hand Freud to a new Forrest, but when I got the Frued it was clear that if the set had been used at all it was only a time or two. It was clean and a visual inspection did not show any defects either in the carbides, and not even a scratch or smudge in the printing on the blade plates. Even the inside of the case was clean as a whistle. The whole thing looked brand new.
Freud SD208 - This was my first dado set and it did an ok job for a long time. It is a little difficult to compare to the others since for almost all of it's life it was used on a Craftsman contractors saw. The arbor was slightly undersized and it took me quite a while to find out how to get flat-bottomed cuts with it. Despite having two of the top rated dado sets on the market, this one is a keeper too. But it is now relegated to those dadoes that I need to make in "junk" wood. You know the kind. You just want to make a whatzit so you go out and find a piece of scrap 2x4 that has been laying in the dirt for a while.
If I had to decide between the Forrest and the Freud, I think I would give the nod to the Forrest. I hate to say this, but I think the main reason is probably because I paid $220 for it and the Freud was kinda-sorta free (I had already paid for the PM66 and then the guy said "you may as well take this too."). But if something happened to the Forrest I would be quite happy with the Freud.
Wayne