I have had 2 of these built for some time now. One has been running on my 11G tank with an Eheim 2213 for a few months, the other is built for my 65G with an Eheim 2028 but I have been too busy (or lazy) to set this one up. I am using Jager heaters, upside down. I think it was a 1" cordgrip. Temperature control and CO2 levels are both great.
The only problem was for the first few days my temp would fluctuate quite alot. It would shoot up to 90 then drop to 70 and continue rapidly changing. However, after a few days this stopped and temp is now spot on.
One thing that is good to have with this is quick disconnects before and after the reactor, just in case the heater needs to be replaced.