I'm in the process of finishing a large install using 4 water to water heat pumps and a back up boiler, all of which heat an 80 gallon buffer tank. HBX will control the whole works. I'm happy with the HBX concept so far- easy to install and plenty of programming options.
I've been using them quite a bit for the past 3 years, just finished a small commercial water to water project replacing a hacked together ddc system with an ECO-1000 + EXP-0100 expansion module + 2 ZON-0500 zone modules + 7 THM-0200 thermostats controlling 2 5 ton ClimateMaster heat pumps & 2 existing Super Hot propane boilers. Works great! I like how you can set the heat pumps to be disabled at either a maximum buffer tank temperature or a minimum outdoor temperature or a combination of both.