I've been (mostly) migrating our home over from ST to HE. We are at about 120 devices, with some pretty extensive home theatre integrations, pile of HUE stuff, and some Webcore stuff running the pool solar heating system. I bought a C7 hub a year ago and played a bit but the sheer volume and time suck nature of a migration had me deterred.
Then ST announced the Groovy thing.
Over the last two weeks I've migrated everything over including all the pool automation stuff (old to new instance of Webcore), locks, kitchen induction cooktop/fan automations, all our main floor lighting, all the garage bits (including door automation) leaving only the home theatre bits in ST. 90% of the lighting is HUE, so I can control these via HT or ST.
All of the ST Zigbee devices were fast and painless to reset / rejoin HE.
Figuring out how to set up Motion Lighting Automations, integrating disable motion/lighting scenes options via a few Aeotec mini motes ate up a bunch of time but are working well now.
So it was a time suck as expected, but I'm quite happy with the result. Bulk of the time, was spent figuring out how to reset devices (especially older Zwave stuff) and in some cases removing them ( door locks) to include them physically close to HE. I had some excellent assistance from @christi999 who essentially wrote new drivers for some older pool sensors. based on the Fibaro FGK-10x door sensors with DS18B20 external sensors.
I'm pretty tickled with the performance on HE hub 
