I recently switched from a pair of beat up Tackla 9000 airs to a Supertacks girdle based on glowing reviews here and elsewhere. It is a little early to tell if I am going to keep using it vs going back to a new pair of Tackla 9000's but I will definitely agree with previous posters regarding size - it definitely fits snug/small to size. Go a size up if in doubt. I'm in a large and debated going with a medium when I bought it, but I am very glad I went with a large. There is no way I could fit in a medium. I have 2 pairs of 9000's actually - a size 48 tall and a size 52 regular. The 48 tall is somewhat snug and the 52 is slightly big. I would guess a 50 would fit me perfectly but they were hard to find up until recently. For reference I am 6'1", 195.
As far as the Supertacks girdle goes, it is much more protective especially in the tailbone and lower back vs the Tackla 900 pants, but it is certainly a different feel coming from the traditional Tackla pant fit. I've only played a few games in it so far, and if what the previous posters have said is true it should start to break in and loosen up. It certainly seems more restrictive for now vs the loose 9000 pant feel. I also cut off that top "tab" above the beltline. It was incredibly annoying.