Hi Roger
I have played on these a couple of times. It depends on what you want to use it for. They have a practice mode where you can hit balls down the "driving range". This should give a fair representation of distance for each club which will obviously help with club selection. It also seems to fairly accurately predict the ball flight, fade or draw and also height, so it gives some useful stats.
When i used it, the main purpose was going out with friends, so after a few beers, i wasnt that bothered where the ball went!! Good fun though.
A downside of using the ones in the city is that you aren't using your own clubs but i'm guessing you wont have that problem if its a club near you. Also hitting the ball off a mat, has never really appealed to me.
As a guide the ones in the city cost £30-40 per hour depending on the time of day and some have a video camera allowing you to assess your swing.