I bought a pair to try last year. Previously I was riding MKS Urban pedals and straps. My thoughts:
I bought some used 2 sided pedals from my local CL. SPD on on side flat on the other. Then I went to REI and found a set of shoes that fit in their returned goods section. Start out with 1 foot clipped in and the other on the flat side. Ride around and practice clipping the foot in and out. Switch feet. Rinse and repeat. Do this until you become comfortable clipping in and out. Go for some real rides this way. Eventually it will become 2nd nature.
As for the before and after? Not that much of a difference. The clipless smoothed out my pedal stroke some. Less pushing down and more circular motion. Not pulling up, though. More comfort and less pressure on the soles of the feet. Beyond that, not much else. It didn't suddenly make me a climber. It's enough of a difference though, to make me stay with them on a bike or two.