Got my first SPDs at the age of 15, now 17 years later I still won't ride without them... Road, offroad, even the 4-mile sprint to work, I just hate not being able to pull up on the pedals and don't feel safe on flatties.
But then I'm not really much of a downhiller or jumper so don't ever really go fast enough to crash! Only had one instance of falling over still clipped in and that was in the first week, after that and since then it's just second nature.
Good pieces of advice I got were:
> On road, if coming up to traffic lights or a junction where you're going to need to stop, unclip before you come to a full halt.
>When you first get them, clip in, ride along on grass, stop, and allow yourself to fall over sideways with your feet still deliberately clipped in. Once you realise it isn't a big deal, the fear of not being able to unclip goes.