it can be tricky striking a balance between making sure you don't spend too much money and making sure that you are displaying your advert enough. We have found search network is better than display network.
The main problem I've had is my quality scores being too low to show the advert. If you have a low quality score, you pay more to have your advert shown. So, someone with a quality score of 9 could pay £1 to have their advert shown at the top, whilst if your quality score is 2 or 3 you will either not be shown at all or if you are lucky, pay £4 to be shown somewhere in the list! Also, historic poor quality scores have an impact on your quality score going forward. So, basically you can pretty much spoil an account by having poor quality score at the start, as far as I can figure (anyone else confirm this?)
I've found Bing adverts to be cheaper and more effective, tbh