I've used Destinology for 2 years now. Basically I find a cheap flight or holiday, then ring up or email the person I deal with at Destinology, and they have always succeeded in finding a cheaper deal for me.
That's impressive given the lengths I go to, to find the best deal in the first place.
The person I speak with gives excellent personal service at all hours of the day and at weekends. If he's not a work, he sometimes still emails.
Personal service and deal finding like this are hard to find these days.