John,
your first question: i have 4 bikes and on 3 i have clipless pedals (Speedplay Frogs) and the 4th i have old clips/straps. it is an '89 MTB (Stumpjumper) and my "junker" bike that i ride in the city maybe 4 times a month when i either want to wear street clothes or park somewhere where i don't want to leave my other bikes...

about 5 years ago when i switched to clipless for my racing and recreational riding i had maybe a little extra difficulty getting used to "twisting" rather than "pulling back" to exit the pedal, but i learned it without much problem. (just tell yourself over and over in the head "twist" and practice so in the "emergency" you twist)

i then rode about 4 years commuting daily w/ clips/straps and doing recreational riding w/ clipless and i had little problems switching and can in seconds switch from clipless to clips w/ straps.

as to being able to wear street shoes with clipless pedals, there are 2 options:
1) 1-sided pedals with clips on one side and regular platforms on the other. i have never used them, but many of my friends use them on their commuters and are satisfied. obviously you can't double-sided click-in and sometimes have to flip the pedal, so not ideal for racing, but commuting OK --- i have seen these usually pretty cheap as they are usually not "superlight" and targeted more for commuters ($30 to $50?)
2) they make these clip-in platforms that you can buy as accessories and clip into the clipless pedal to provide a platform pedal for use with regular shoes. advantage is that when they're off they are only clipless pedals with no extra weight or change. i would buy these except i use a special type of pedal Speedplay Frogs and as far as i know they don't make any --- i guess i have old cleats and i could make some myself, but i have been too lazy.