I have not played with the thumb-stud models, but I have bought both a 550s and a 555s in the past two weeks and LOVE them! These are the first new Benchmade designs that I have bought in several years, and the first knifes that have been able to dislodge the Spyderco C48PS Tim Wegner as my main EDC.
On both knives the blade travel is smooth and wiggle free, the handles are very grip-able, the clip tension is snug but not too tight, and the axis locks work smooth and securely. Even on the smaller 555s, the flare to the front of the grip and the multiple grooved/ridged surfaces make the grip very secure for heavy cutting/thrusting/slashing. Edges on both blades were hair-popping sharp out of the box, and touched up good as new after all my playing with them with a a few strokes on my Razor Edge folding steel.
The opening oval hole on the 550s took a little getting used to because my thumb kept wanting to bind into it, but i got past that after the first day of playing with it. Seems the wide part of the oval was big enough to let a big part of the tip of my thumb down into it, and then as the oval 'rotated' as the blade was opened it would bind slightly on the sides of the tip of my thumb. This did not make it hard to open, but slowed things down when I would try to flick it open.
The 555s is so light and small that I forget it is clipped to my pocket, but it cuts like crazy and is very comfortable for either strong or weak hand usage. I have very little usage of the first knuckle on my left index finger (old BAD cut), and this is one of the few small knives that I have played with that I feel completely comfortable opening and closing one handed weak handed.
I prefer the opening oval and the wide, blunt blade of the sheepsfoot design, but if you prefer the MDP or a thumbstud, or both, then I think you will be very pleased with the 551/556.
Sorry to be longwinded, but I am VERY impressed with these knives!

I will probably buy a spare 555s and a 555 and send one off to Mike Sastre to have a 'kneck' sheath made for it.
P.S.
Don't be worried about the zytel handles, these little guys have full steel liners/inserts in them.