You get what you pay for - again
Disclaimer: I sell VoIP and VoIP capable PBXs (But only to businesses)
The biggest issues I've seen are cheapskates who see VoIP as a cheap solution to their telephony issues. It can be cheaper than traditional telephony solutions, but that's just part of what VoIP is all about - Flexibility is what I mainly try to sell to people.
But no matter what, if you want VoIP to work for you, then you absolutely must have a good, reliable broadband connection, and for a business, that means that you should be prepared to pay a few pounds a month more for a good, reliable connection from a reputable ISP, and maybe even consider a 2nd dedicated ADSL line purely for VoIP.
I've (sadly) seen people spend more trying to cobble together a VoIP solution using residential quality broadband and equipment than if they'd stuck to their old fashioned BT wall socket.
Interesting this VoIP question comes up today. On Thursday last week, the Internet Telephony Service Providers Association (ITSPA) celebrated their 3rd year in existence, and held their first awards ceremony. I was there along with - well, I'm not sure as I'm not good at counting - 150-200 other people? These were people representing all aspects of the VoIP industry in the UK - big names (Cisco, BT, Magrathea, Gradwell) as well as some lesser known ones, (Drogon Systems) but all with one thing in common - a vested interest in making VoIP in the UK work.