Choosing the right person or company to provide the right service isn't a difficult decision.  If you want quality products, quality service (and the smallprint inclusions ie. liability insurance, waranties etc), you choose a professional.
For a lot of people though, the major deciding factor tends to be price.  Its unfair to criticise someone for wanting to find a better price, but the saying 'you get what you paid for' is around for a reason. 

As a professional installer I am often sent to premises' with the aim of upgrading existing CCTV systems.  After a quick assessment (noting to myself the inferior quality of both equipment and installation) I will ask the stock standard question: who did the your original install?  This is usually met with `Umm, this guy I know,' `Some alarm company who said they did cameras,' or `The same guys who put in my tills.' I've even had someone say the guy who did my sign.  These answers are usually what has led to my company being called in. 

If you go for someone on the cheap or someone who assumes they know what they're doing it leads to all sorts of complications.