I have a Raspberry Pi (model B+) based system that's primary function is visually alerting barakta to things like doorbells and fire alarms, but it measures the temperature and occupancy in each room and controls the heating as well.
The main issue I've had is that the Raspberry Pis aren't long-term reliable...
At some point over a period that can be as short as several hours, but is more usually several weeks or months, they will get into a state where a) any attempt to play audio blocks and b) write operations to the SD card block. Normally cycling power *without* attempting a clean shutdown (which pretty much guarantees FS corruption once in this state) restores functionality, but sometimes the filesystem is corrupted and the system fails to boot.
This is basically impossible to troubleshoot, as it's so hard to reproduce. Any mention of unexplained crashes on RPi forums results in a deluge of cargo-cult solutions involving different power supplies and alternative brands of SD card. My gut says it's somehow related to the HDMI output, but I can't get any further than that. At this point I've settled for detecting the hung state (by the audio player thread of my high-level application failing to return in a timely manner) and bleating for a human to come along and cycle power. Which is suboptimal, but since the network and GPIO aren't affected, the main functionality of the system continues working.
In other words, if you're going to use a Pi, you're going to need a) a plan for what happens when the root filesystem becomes corrupted and b) a hardware watchdog timer (the Pi's internal watchdog isn't up to the task).
If there's any risk of freezing conditions, I'd recommend leaving a traditional thermostat in place, wired in parallel with your control relay. Set it to 5C and forget about it.
I'd strongly advise against rolling your own solution if you won't be physically present to debug it, at least for the first year or so, until it's proven reliable. It's also unlikely to be cheaper than a commercial solution, unless you write off the cost of your time entirely, and cut corners with things like enclosures. Once you've put a Raspberry Pi, a custom circuit board, some power supply components, a robust 1-wire interface and a sensibly isolated mains relay in a box you're probably up to about the cost of a Hive anyway. I think my BOM cost for each module was in the £200 range, but there's a lot of extra hardware there that isn't related to heating control.
