I have worked, and still am currently working for WH Smith.
Have been there since September '05, so will be happy to answer any questions that you may have.
Pay varies depending on the store. Larger stores have larger budgets so tend to be able to offer large salaries/pay rates, depending on the number of staff they employ.
Because i work in a relatively small store, all the part-timers get the same wage depending on position and age.
Sales assistants at 16-18 get £4.81 an hour - that's the minimum WH Smith have to pay you due to policy.
Same position at 18+ is £5.13 per hour.
With me working in such a small store, the jobs are varied, and extra responsibilities are given, but usually revolve around what most tasks would be in any store, which is stock replenishment, serving customers on a till, tidying up etc.
Sometimes you may be required to help with promotions and things, but as i say it depends on size of store, which area you are working in etc.
From a personal opinion, the job has served me well these last few years. The pay is average, the hours limited in my case due to tighter store budgets, and sometimes it can be boring, but you get to meet a variety of people, and it looks good on your CV.
Any more questions feel free.