Halifax county here in Nova Scotia merged all of the independent FDs (as well as all municipal services, including policing) into one government system and one fire department back in 1996. I know there was a lot of resistance back then from the departments losing their autonomy, and there may very well still be some "bad blood" about it to this day, but 15 years has given them a lot of time to work out any issues. I know the whole point behind not just merging the FDs, but all of the local city/town governments into one regional municipal government was reduction of costs and as far as I know that goal was met.
As for the fire service specifically, a couple of smaller sub stations were closed, some stations replaced (which probably wouldn't have happened before), a lot of new equipment for the more rural stations that again they may not have been able to afford before. In total there are 59 stations serving 380,000 people, with a mixture of stations staffed 24/7 in the cities, a mix of career and volunteers in some areas, and 100% volunteer stations in other areas.
Some other cost savings that I would imagine, although I have no stats to back this up, one unified dispatching system (in the case here both fire and police county wide), and better use of apparatus - there's probably no need for every single station to have an aerial unit or a tactical/heavy rescue unit.