I got one last year - my mum was really pleased as she said it gives her peace of mind knowing that if something were to happen to me, then they'd find my ID which would at least give me a fighting chance of surviving.
One quirk of the NHS is that at present, medical records are kept only at your local hospital and doctor's surgery. I work up in central london, several miles from where I live. if something were to happen to me while i was out working (my office know and we have a doctor in the building, but i'm often out and about by myself), no one would be any the wiser until they'd spoken to my local hospital or got a CBC done. So my bracelet tells them everything that they'd need to know at short notice:
Name - date of birth (i have the month written as July to avoid any confusion between british and american date formats)
ITP - Low platelets (medical staff don't need a quiz - tell them what's wrong with you!)
ICE my mum's name and mobile number
I also have both my parents listed in my phone as ICE.
I wear mine all the time, and i got a free dogtag which lives on my car keys - admittedly my count has been sub-50 for quite a while, but even if i'd had a recent good count, i'd still be inclined to wear it for just in case they'd dropped without my knowing. and remember that there are drugs which can lower your platelet count/thin the blood etc - you don't want to be prescribed those without good reason! and you might have good counts because you're being treated, whether it's with steroids, IVIG, rituxan etc - but doctors will want to know about those too! Make things easy for them - the more info they have immediately, the better and the quicker they can treat you in the event of an emergency.