I have used both the standard seals and the user-replaceable ones. The UR ones work as advertised -- the seals can be swapped out in about five minutes. The neck seal requires a moderate degree of profanity, the wrist seals need a tool, but they both work. If you plan on doing a lot of traveling to dive, being able to change a seal is awfully nice, because unlike a wetsuit, a dry suit with a torn seal is useless. Many places you might dive may not even have dive shops that sell or service dry suits, so a torn seal could kill an entire week of diving. (I had a zipper go in MX, and had my former cave instructor not lent me a suit, I would have been beached for the entire week!)
However, there are three downsides to the UR system. One is that the neck ring does limit arm mobility, especially if you are small-framed. Since one of the utter delights of the Fusion is mobility, it's hard to give that up. With my full rig on and my thick undergarments, I have a little trouble reaching my dump valve, and trouble clipping things to the left front d-ring. I've learned to finesse it, but it's definitely more difficult. This is not a huge issue for single tank divers, but it's a safety issue for people using doubles.
Second, you will have the wrist rings on all the time, and they are bulky and annoying to get in and out of your BC. A small item, but it's real.
Third, if you want to use dry gloves, you will be limited to systems that are compatible with the UR seal setup. To my knowledge, that comes down to the two SiTech systems. The Glove Lock is VERY large and bulky, and requires purchase of an additional part to make it compatible with the wrists. The other system I have used and don't care for at all.
At this point, I have the neck ring on both of my suits, because they came with it and I haven't had it removed. I would probably keep it for my warm water/travel suit, for convenience, but it's the home suit that gets the most use and requires neck seal changes most often
I have the wrist rings on the travel suit, but not on the home suit, because it's never used without dry gloves, and I don't have to replace wrist seals that often.