I guess that I want to call it a 4.5, but only because it wish it was 75 or a 100 meters long, not just 50. Nevertheless, I just finished my first test spool of 100 D and loved it. It twist tight and strong, or unwinds to very flat. When unwound, the split thread method of dubbing, etc., works well.
Like all GSPs, it is super slick, and will cut bucktail, etc., so look for larger then 100D for bucktail and similar use.
When it is back in stock, it will be worth getting. Just get more then one spool at a time. 👍