Verdict: Not really...
SPF stands for 'Sun Protection Factor', referring to its ability to block UVB rays. It is UVB exposure that is responsible for sun burn and skin cancer.
A SPF rating indicates you how long you can stay out in the sun without getting burnt compared to how long you could stay out without it. For example, if you burn after only 15 minutes in the sun and you use a SPF15, you will be able to stay out for 225 minutes (2hrs 45minutes) before you burn.
So although a higher SPF offers protection for a longer period of time, the actual protection that you get within the specified time frame is no better than that of a lower SPF rating.
It is important to remember that as a SPF only protects against UVB exposure, you need to look for a sunscreen that also protects against UVA exposure (UVA exposure causes premature ageing). I recommend using the Decléor Aroma Sun Expert Summer Oil SPF30 150ml (£24.00 decleor.co.uk / $49.92 feelunique.com). Not only is this perfect skin-protector an oil, it also comes in a spray bottle, making application easy and speedy - just want you need when you're in a rush to soak up the sun! It also leaves skin deeply nourished and soft, which is an added beauty bonus.