Pay-per-click (PPC) v Search Engine Optimisation (SEO) is a really long debate.
Speed & expansion - PPC allows you to compete with any of your competitors within the search engines immediately and on any scale. Whether it's cost effective is indeed another matter.
Geo targeting - .
Constant monitoring - PPC campaign's need constant monitoring. Leaving a PPC campaign can be very dangerous. For example what would happen if you accidently set your daily budget to £1000 rather than £100? A very expense mistake could be on it's way. The key to a sucessful PPC campaign is the constant testing & tweaking of your campaigns.
Awareness - Internet users are much more savy than they were 10 years ago. They are more likely to know the difference between organic and sponsored results that appear in their search results.
Cost - You should ever assume that you will make money from PPC, more often than not you'll make a loss when you start off experimenting with what does and doesn't work. PPC can be highly competitive and in some areas the profits can be slim.
Click fraud - Fraudulant clicks will cost you money & reduce your ROI. Click fraud is however in the top tier networks such Google Adwords.
Free -
Long term sustainability - For most companies or organisations, I recommend splitting you time equally between both PPC & SEO.
Slow - SEO is a long and often dark journey. It's difficult for anyone to invest in something that won't give you any clear benefit for 12 months - sometimes much longer.
Algorithm changes - Search engine optimizers are always fearful of the effect that shifts in the algorthms will make to their rankings.