I've been doiing loads of reading on HERMS and advantages/dis-advantages because I have some extra copper that I was trying to find a project for. There's quite a lot of theories on temp control, loss, gain, etc. The basic gist of what I take from it is that there is an associated heat loss due to the lines & pump being at ambient but it is not significant due to the speed of the fluids (once the lines & pump warm to the recirc temp) because those pieces are not very efficient heat dissipators. The reason I decided against HERMS, at this point, is because temp control is not necessarily better due to the fact that temp changes do not occur instantaneously throughout your mash. The output of the HERMS will normally be hotter than the rest of the mash and it takes a while to propogate throughout.
For minor temp adjustments, this would be fine, but I was most likely attempting to make up for any mash tun/HLT shortcomings and I didn't want to design in complexity to overcome something that has not been a big problem for me. The huge advantage to this system is that your mash runs extremely clear from the start of taking runnings due to the constant, or near constant, recirculation.
So now I have loads of copper that I think may become part of a tower cooling system once I build my coffin box for a draft system.