You must register with HMRC within 3 months of starting SE or face the prospect of a penalty for not doing so.
Apart from being able to offset the expenditure wholly attributable to the work (including that you don't have to pay commuting costs if you work from home), there isn't too much difference in what you end up paying in tax and NI, compared to being an employee.
Make sure that pay rate you are offered factors in enough for employers' NI, and benefits not paid by the employer. In other words, don't get sold short.