If you plan on using it and then paying it off straight away ( your next pay) then its ok, if however you plan to use it to pay something you dont have the money for and remain overdrawn from month to month then its a bad idea.
Look at it this way, you get an overdraft for £100, great you initially have an extra £100. You use the £!00 and your immediately £100 overdrawn ( obviously )
The following month you get paid and your immediately £100 worse off because £100 has just cleared the overdraft - great if you can afford that, if not then you just end up using your overdraft again and again and again and getting charged for its use.
In my opinion dont get one - save the money up.