Had one since August, it's been spot on, had to reboot it once but it does exactly what it says on the tin, gives full signal at home. Like others, had to phone them, took about 20 mins and fairly painless, just explained I had zero signal at home and that the Three In Touch app (released literally the week before) was still too buggy and not reliable enough - one turned up the next day via DPD.
In the more recent T&C's though, you need to give it them back if you leave Three or they charge you £75 - you do NOT own the device, you lease it from them, as you're not legally allowed to broadcast the signal at home without a license etc - i think the earlier customers got away with not been charged, hence theres lots on eBay.
Can add up to 30 numbers to it (if your friends and family often visit etc)