They give you just about all the advantages of a dedicated server for a fraction of the price. Being able to do what you want ,anytime you want eliminates many issues. You can also run as many domains as you want so the price can be less than some shared hosting.
This article has some tips for shopping for a host:
E-commerce - Getting Started Guide
Cheers for the advice and the guide but the problem my clients will throw back at me is they are all getting much more aware of the price of hosting prices (they're advertised
in all the magazines) and, therefore, me suggesting they pay around £400 p.a. rather than around £75 p.a. just makes me look like I'm robbing them.