I have done PoA's for both my Mum and Mother in Law. In my Mum's case, she is absolutely fine. The PoA is there 'just in case' as it saves a monumental amount of hassle if anything should happen to her. My MiL unfortunately has dementia, so hers is 'in effect'. When you do a PoA, it costs £130 (I think) for each one (there are 2...'Property and Finance' and 'Health and Welfare'), but you can have as many attorneys as you like for the same price, so Mum has 3, MiL has 5. They can be free if you are on Income Support or similar (info in the pack from the Office of the Public Guardian).
As to setting them up, my wife is her Mums principle attorney, so had to go to the bank with the PoA and her passport (as ID). Took about an hour, but she now has a bank card and cheque book in her name with 'Power of Attorney' after it.
Martin Lewis was on Jeremy Vine (Radio 2) last week and said everyone should have a PoA. Its all well when you see something coming (like dementia), but you could be run over by a bus tomorrow and paralysed or something. If the PoA is in place, it only takes a few days to get 'power' transferred.
If you want to do one yourself, go to the Office of the Public Guardian website and order a pack (its free!) and there are a couple of 'how to fill it in' books in the pack which are actually very good. It took me about 4 hours to fill in the forms for Mum, then had to run round getting signatures from Mum, the attorneys, an independant witness (ie not a family member) and a 'person to be told'...this is a safeguard so someone can report if the think the family are forcing a PoA on a person to get control of their affairs. Its not as daunting as it all sounds!!