If you've not tried them, you wont realise that most of what is written above is nonsense
being able to use your legs on the upstroke of the pedals makes life a lot easier up hill. In the event you end up in trouble you tend to end up unclipped anyway. Walking is very shoe dependent, but there are plenty of shoes that are just as easy and comfortable (and of a similar style) to skate shoes. So much so that I often just wear my SPD shoes to walk around the shops or go to the pub...with no bike involved.
Cost, unless you ride cheap resin flats like those on super market bikes, you are probably already spending as much or more than a set of SPD's can cost. The shoes are an additional expense, I shall grant you that.
I won't lie to you, whilst getting used to them you will have the inevitable SPD related injury. Most often purely an injury to pride that occurs once you start to get confident in unclipping, pull up to a set of lights or a slow stop, then forget to unclip and end up in a heap on the floor looking a tit. But this phase tends to be short lived, nothing teaches better SPD use than embarrassment.
[size=67]Where we're going we don't need roads!
Under cover operative of the YVMBAPFS cartel.
The Pace fleet:
16" RC100/ 21" RC100/ RC100 Trials prototype/ RC200 F5/ RC200 F8/ RC250 Trials/ RC300 Gunmetal/ RC300 Silver/ RC500 F1 DH