Proponents of higher education say that postgraduate degrees such as an MA, MSc or MPhil give graduates a thorough grounding in the subject and also more opportunities to undertake independent study. This is important for any career where you will sometimes be left on your own to solve certain problems before discussing your ideas with the rest of the team. Also, a postgraduate qualification will no doubt differentiate you from other people who have completed bachelors' degrees.
However, there are careers such as banking and retail, which encourage learning by doing. This means that exposure to the industry at an early stage will help new graduates understand the key areas to focus on. This is not to say that graduates in these areas do not embark on further study, but there is also the chance to flourish without years of study behind you. Nowadays there is the odd problem of being over-qualified and running the risk of not being hired since you are deemed to have too advanced qualifications.
It is worth noting that there may be a compromise. This might not be the easiest option but it is worthwhile in the long-term. There are certain careers such as Accountancy where graduates join an integrated training programme to gain work experience while studying for a professional qualification. Although it will be difficult to make time for study, with discipline and hard work, there is no reason why this can't be a success. Companies usually pay the training costs so that is another incentive for this route.
Overall, it is well worth considering postgraduate qualifications, but it is difficult to say exactly how much value they add, since it depends on the particular industry. It is useful to read the requirements for different jobs and speak to people in the profession to understand the path that they took. Finally, you need to consider whether you will be happy jumping in at the deep end or taking more time to consider your options while gaining a further well respected qualification.