I’ve never been tempted to buy one for home, but from the experiences of friends who have:
– cheap ones are rubbish. Treadmills need weight to be stable and work properly, and that inevitably means cost. Anything decent is likely to weigh 120+kg, and thus NOT be something that can easily be moved around and put away when not in use.
– equally, consider space carefully. Ones big enough to run on comfortably need a fairly wide and long deck. They don’t look too big in a gym but can look huge in a domestic setting.
– don’t even think about putting it upstairs, or buying one at all if you have a non-ground-floor flat. Weight of person + weight of treadmill will cause lots of problems for the celing downstairs (not to mention the structure of the floor!) plus loads of noise when it’s being used. Garage is ideal, especially as it tends to be cooler so you won’t be sweating buckets.
Personally, if I wanted a machine for doing cardio work at home I’d get a Concept II rower. Far more flexibility about where to put it, easy to get out of the way, replicates doing something you can’t get by walking out of the front door.