I've set up 2 Hive systems for friends, the set-up did not go smoothly both times, i think the issue was that I was trying to follow the set-up instructions but in reality I think if you power it all up they seem to connect and sync up on their own eventually. The main downside for me is that you have a box to connect to the modem, and the reciever and the thermostat whereas nest just has a thermostat and a reciever so thats one less box to have powered up near your router.
I have a Nest and I'm happy with it, it did die recently though (along with many other Nests that have a common issue where the wifi chip dies) and I have to say the service from google was brilliant to say it was well out of warranty. As soon as I found the issue I went to the website, chatted live and within 10 minutes a new one was on its way, the next day a new thermostat and heatlink was at my door (now I have a spare heatlink also).
One downside of the nest though is its power requirement, for it to look neat and tidy on the wall you either have to chase the wall out or in my case, drill through from a cupboard on the other side of the wall. Otherwise you have an unsightly cable running up the wall unless you have a table to sit it on with the stand. The Hive being battery powered does not have this issue.