The SAS are a highly motivated elite group, peerless, adaptable and unorthodox, no other country has such an effective force. They are are carefully selected on the basis of mental aptitude rather than physical prowess, the selection process is arduous physically but challenging mentally. They come in and out of prominence depending upon political circumstances often in the past having to justify their existence and funding to the British Government, having to fight for survival as a regiment in peacetime let alone in actual operations in the field.
The SAS culture is unique, the complete antithesis of conventional, regimental military life, they are self disciplined, realistic, democratic, pragmatic, adventurous, independent, eccentric and refreshingly bullshit free. The SAS were particularly effective in covert operations in Malaya and Borneo where they countered the influence of insurgency by winning the hearts and minds of local inhabitants, offering medical treatment, listening to their concerns and responding sympathetically to their basic requirements. The reality of the SAS is most of their time is spent tediously gathering intelligence, in hostile uncomfortable environments or training, despite the glamorising of the legendary skirmishes and battles they have fought over the years. So on that basis they were and are effective, no missile can distinguish friend from foe, technology and sophistication will only get you so far, no country bar Japan has ever been bombed into submission. So yes, special forces have been effective to varying degrees depending upon the nature of the conflict.
All in all, for a group of soldiers that are not ideologically driven and not overly paid they are surprisingly resolute. Having said all that the mystique of the SAS, the derring-do and the ventures, often eclipse the dubious ethical nature of their deployment, which is frequently to brutally suppressed militant activity of an ideological nature, wheresoever it should adversely affect British Post Colonial interests.
Ultimately they are bad ass in ways Americans could only dream of, they are to Soldiering and gun play what Brazil is to soccer.