Officers at the Pentagon call them a "paper tiger." Even in a sneak attack, they couldn't advance 50 miles into Iraq against our ground forces and air power. They have second-rate weapons and use WWI ground force tactics. You might want to "get out and read more" yourself.
IMHO,our military is stretched too thin right now to do the kind of job that would be needed in Iran. Then of course you have all of the domestic appeasers, etc. who would go all out to stop us from having another front in the war on terrorism. Lastly, even though regime change could work, it needs to come internally - even anti-regime forces are probably nationalistic and might not necessarily welcome our direct military involvement.