I having been brewing on a 25 gallon 3 vessel direct fired rims system for the past 5 years. I recently relocated and don't have the room to set that system back up, so I decided on a Grainfather to get back to brewing until I move again.
I have just one brew down with the GF, and while it is very easy, it is also very slow. I know it is unfair to compare to my gas system, but going from a system I had down to 4 hour brew day, to over 7 hours is a real let down. I will note that I heated up my sparge water in the GF, then drained that into a round 5 gallon cooler, and heated my strike water. Next time I will heat all of my water, then drain out the water I need to sparge into my cooler.
In my opinion the heating system is very under powered. I read some of these heat times and I am surprised. It took at least 90 minutes to get to a very slight boil. I guess this could be due to altitude. I am basically at sea level. I cannot get a gently rolling boil (indoors) unless I keep the lid on! Granted, I did have a full 8 gallons in the GF.
I will definitely be looking to supplement the heat source with something like a heat rod. I also just made a jacket for it from reflectix insulation.