Another big no here. On exactly the same tank as I have now I ran a heater cable for about 3 years until it broke and blew all of the house fuses. Never bothered to replace it and to be quite honest (prior to the lighting upgrade and pressurised CO2 addition) I noticed absolutely no difference in growth rates, nor any anaerobic areas forming in my substrate. This was a good 5 years ago as well, with the exact same home made substrate. I'd rather spend the money on good ferts or CO2 than on a useless heating cable. All they do is act like a normal but lower powered aquarium heater, and if you don't link them to a thermostat can cause real overheating problems on hot days. More trouble than they are worth, and of absolutely no benefit either to plant growth or to the prevention of anaerobic pockets in a substrate (which in a well planted tank never get a chance to form anyway).
I used the Ehiem heater cable in my 90 g. and after the first year it stopped working. I just ignored it ( I did not want to take the tank down). I went about 10 years without it and the plants did just fine. When I did take took the tank down I found the break - not caused by trauma, either.. I would never use it again.
Mine's a no aswell. I nearly bought one - was going to go with the full Dennerle setup where you have to use the heater cable with the deponit mix, but found this website just in time! . On another note, I find it annoying that some company's advertise a heating cable as an essential piece of equipment, when it's definately not.
Also have one in my tank that I installed about 2 and a half years ago at the recomendation of the LFS, it stopped working about a year later and its still there burried in the gravel, pops up once in a while when I take plants out. Maybe they don't do a lot of benefits but don't see them causing any harm either and the flow of the heated water through the gravel could be beneficial for the bacteria within it for filtration purposes rather than plant wise.
I`ve never used them and i`m about to start another tank so looking over that report and the general response on this thread i think i`ll give it a miss.
Walking into a LFS here in Swindon last week the guys in there were besotted with them and made all kinds of threatening comments that nothing would flourish or grow without one. They also condemed EI dosing suggesting it forms a more complicated balance with nutrients (oh if only Clive was there at the time)!. After 5 minutes of that i grabbed a couple of pots of HC cuba and walked out. I know we all have different opinions because different solutions work better in different environments but their was no need to try and commercially push products so hard surely!