SIPPs offer the same great tax advantages as other personal pensions but provide greater investment flexibility, allowing you to choose where you invest your pension. Therefore the SIPP allows you to take control of your pension and manage it at a level that suits you.
Some Sipps have large annual fees making them only suitable for the extremely well off, however the Vantage Sipp from Hargreaves Lansdown has no set-up fee and low annual fees (in many cases no annual fee) meaning more of your money gets invested.