As mentioned above,
just recently got my rackets restrung from BG80 Power (25 lbs) to BG66 Ultimax (25lbs),
the BG66 Ultimax feels like metal although it's strung at 25lbs (has the same feel as 28lbs for other strings),
hmmm still not quite able to handle/control it yet,
do you guys feel the same?
Many (many) years ago (gulp), I used to use BG65. As I had my own stringing machine then, I tried out BG66 which was relatively new then. I found it very difficult to play with.
Fast forward to latter years, I find BG66 quite good to play with and string it at a higher tension than the first time I tried it.
The difference could be a change in the string's material over time.
But I believe the major difference is my technique over time. The first time I used BG 66 my technique was really quite poor. Can still play matches and competitions but using too much strength and had not learnt to use much finger technique. After getting some formal coaching and a lot of technique practice, I found BG66 a great string to use.
Whilst I haven't used BG66UM, my experience may give people something to ponder upon.