I just went from long time direct fire MT with circulation pump to a HERMS system that lives in my HLT. I did my first brew on the HERMS last week, and the jury is still out.
My direct fire system had me constantly turning on and off the burner to maintain temps and avoid scorching (I never did scorch....good false bottom), but the temp fluctuations always bothered me.
My new herms system seems to make it easier to maintain a given temp, as the temp changes slowly....however, I figured that I could raise the temp to mash out using only the HERMS system, but it took a little longer than I expected to get the temp from mash to mash out....I think it is not an indictment of the system, but a problem to be solved through process and procedure....or call it operator ignorance.
In the long run, if I figure out how to use it, I will like it much better....in the meantime, I hope my beer is not adversely affected....check back in a few weeks.