ive bought a few insurence write offs in the past. I do it though because I actually like working on the cars and getting them back to how they should be. Most writes off's have been written of for valid reasons and even though they are cat d or c a lot of them should not go back on the road and really should be breakers (cat b). A lot of write offs in the UK get exported, lots of the 4x4's go off to africa. Some get taken away and repaired and reimported. You have to know a bit about what you are doing and what you are looking at. That being said, there are lots of great projects out there for your average petrol head where the car has had minimal damage and its just been decided for pure cost based reasons to write it off. I try and generally take on bolt on only repairs, and I avoid ones with blown airbags (dash replacement jobs), but I have an excellent body shop I use when I do need to do panel repair. I also keep all the damage of the photos of the car in its pre-repaired state and during the repair for if I decide to sell later on.
whenever your buying a car you should do a full HPI check (the RAC web site one I find is best). You really also need to get under the car and check the structure. Inside the car it doesnt take long to see if the dash has been out. etc...
I think as a general rule if you are not 100% confident with what to look for I would actually advise dont by cars that have been written off. This white GT looks like a car thats been hit hard and poorly repaired. But just looking at the photos you can get caught up in how good it kind of looks on the surface of it and the nice price tag.
however again, if you are confident and you can find an insurence write off or repaired car that you can satisfy yourself did not take any damage to the critical structure and has been repaired to a good standard. then you can save a lot of money. The key is "seeing the damage" because some are written off for like graffitti spray all over them, some for water damage to the electrics and not the engine etc... (a friend of mine bought a 2012 cooper S for 3k as a water damage (real low mileage as well) and all we did was replace the electronic accelerator pedal and the car was repaired) but its not all roses though, you have to be carefull or you can loose money and waste your time.