As others have pointed out there is a distinction between step ins that have a metal cleat on the boot that locks into the binding and bindings that have a swinging high back (Flows & K2 cinch).
There are still a lot of the first step ins (w/ metal cleats) in resort rental shops. While a good idea in theory they aren't very practical because what often ends up happening is that the cleat/locking mechanism often gets snow wedged in causing it to ice over and become non-functional. Don't recommend this type at all.
I have both Flows (swinging high back) and conventional strap bindings mounted on my boards. Things to consider regarding Flows and K2 Cinch:
1) They tend to be much heavier than strap bindings... sometimes almost twice as heavy depending on the model
2) Setting up the Flows so that they fit properly is a trial and error process that you'll have to do while riding. Many people bag on Flows saying that they don't support well or their feet slip out.... but that is because they haven't set them up properly. I own a pair of Flow's high end team binding (with the magnesium base) and they are as stiff as any strap binding out there (but heavier).
3) Once you've riden Flows for awhile and get used to them you can strap in much quicker off the lift... I've been able to kick my back foot in and engage the highback while riding off at times.
In the end it's a matter of personal preference and not so much one of performance... I mostly ride toecap strap bindings. But if the park/pipe is crappy and the only place good to ride are groomers than I take my "speed/freeride" board and Flow bindings.... less time strapping in = more time riding :)
Peace from Mt. Hood, Oregon