Is a Will legally binding?

If it is properly executed, then it is.  However, there are some laws that allow dependants or others, who have not been provided for, to challenge a Will.  It is a good idea if you are not intending to benefit people that you would normally be expected to benefit that you draw up a letter of reasons saying why. 

People can also challenge a Will on the basis that a testator did not have legal capacity or was coerced.  If you draw up a Will using a solicitor, they should consider these matters and do things in a way which should help to minimise any challenges to your wishes after your death.