I have a 550, 551 H20, 555. I prefer the sheepsfoot blade with Spydee hole.
For me, they are lightweight, fun to open and close, have better than average blade steel, excellent handle ergonomics, and traction. Offered in "rust proof" steel, as well.
Some people complain about the prices, "cheap" feeling plastic handle, and the assisted opening spring being prone to breakage. I have been aggravated by poor blade centering on one of mine. Mini-grips are too small for my tastes.