Well, I run a will writing company so I would say this 
but you need to bear in mind that online wills are basically just programs.
An actual human lawyer will ask you all sorts of questions about your circumstances including your financial position so that they can advise about inheritance tax. We'll also ask about the family relationships (eg sometimes clients don't want their child to have contact with other family members, or on the other hand, want to make sure that if the guardian is going to be a family member on the dad's side, that the mum's family can also see the child regularly).
If a child has special needs, the will writer will advise about discretionary disabled children's trusts.
Sometimes clients are on their second marriages and have children from the first marriage which they want to ensure aren't cut out by the stepparent after their death.
A will writer or other lawyer in other words will talk to you, get to understand your priorities, concerns etc and will advise and guide you through the procedure.
No computer program can do as good a job as a qualified, experienced will writer or solicitor to be honest.
Fees can be quite high; when I was in high street practice I charged north of £350 for a pair of mirror wills including VAT. Now I'm freelance it's £170. You may find someone cheaper but you need to make sure that they're fully qualified and trained and a member of the Institute of Professional Will Writers.