I'll try to answer that from a personal experience and view point.
I had ActiveSky6.5 as it transtioned from FS9 to FSX, then ASX and now ASE. I've never found anything that I don't like about the whole transition and ActiveSky family of products.
I also own REX2. I bought it to see if there was anything at all in it that backed up all the excitment that surrounded it. I worked with it for part of a day during some flights and promptly deleted it and went back to ASX by the end of the day.
Then about a week later, thinking I hadn't given REX2 a fair shake, I re-installed it and played around with it for another day or so. Once again after that, I let REX2 return all my default settings and then turned ASX back on and let it take back over all my weather needs.
I still have REX2 installed, but I don't use any parts of it at this time and now with the ASX upgrade to ASE, I have an even less desire to give REX2 another shot.
I haven't canned REX2 completely, that's why it's still on the system this time.
Personal opinion:
ASX/ASE - The GUI interfaces are more intuitive and functional. I also find that in most weather situations that I've flown in the ASX/ASE weather just looks more natural as to what I would expect. Overall ASX/ASE work and look like a favorite tool in my tool box. Reliable, simple to use and it gets the job done.
REX2 - The GUI interfaces are very clumbsy and some like the flight planner and weather view map are not very useful at all for the type of flying that I do. REX2 can make for some really good staged screenshots showing some really hi-rez clouds, but at other times the colors of the sky and reflected light off the aircraft don't look natural at all. I also noticed much more of a cookie cutter repetitive cloud pattern almost everywhere I looked in REX2. I'm pretty sure that some of the clouds that I was seeing were identical to other clouds that were in the area, they were just rotated in various directions to minimize the repetitive effect. I barely ever notice that with ASX/ASE. Overall REX2 seems to be more about eye candy than real functionality. That functionality may actually be there, and I'll go back one day to take another look when I feel like wading through all the eye-candy bloat it's hidden under.
FAC