i use the PF flat out everyday and i can honestly say it doesn't cost me 100 quid extra a month on my fuel bill.
I had to use my other van earlier this week which is a cold system and after 20 minutes using it i wondered how i had managed without out hot so far.
The temp was about 0 degrees. The water was freezing on a conservatory roof. algae wasn't coming off as quickly. The hoses were so stiff it was hard to work with plus my hands were so cold on ally poles i had to keep stopping.
I think its a case of you don't know what you had until its gone.
That will be the last time i use my cold system unless i really have to

I'm also gonna upgrade the cold system to a hot PF by the end of this year.