The camera I use is a Mio MiVue618, bought for about £70 in a Black Friday Sale
last year. I think it has just been superseded by others in the range. It has several useful features not always found on other makes and was a recommended buy. For example, it has an updateable (free for life) speed camera location database and can be set to monitor the lane lines in case you drift off course, and if you are in danger of dozing off, it can also alert you to take a break at a preset driving time. It records time, GPS location & speed, which is recorded at the same time and you see that on screen, while G-force is logged as well, but you need their software for that.
Obviously it's ignition switched, but it can be set to stay on permanently (assuming a permanent power source) or be left in 'parking mode' so that when you switch off its internal battery keeps going for about another 8 minutes while you nip into the shop or whatever.
My other choice would have been a NextBase, but it didn't have the same bells and whistles and cost more. Halfords provide handy video clips from their range of cameras, which will give you some idea of the quality that's out there.
While most claim to be sharp and crisp (a feature of most things taken with a wide angle lens) and HD, both of these claims need to be seen. For example, can it reproduce a number plate registration at a minimum of the driving test standard? Probably not, but as DD says, they do have some work to do to get all that video onto a memory card, so there has to be some trade offs.
I did try a cheap one, but the instructions were incomprehensible and the buttons were illogical. There was buzzing on the sound and its infrared light didn't work, so if you tried it in even partial darkness (under the desk in my case) you just got a black screen. That went back the next day.
Pax - a Tracker unit is something completely different. They are hidden in a vehicle. Some transmit a signal that can be picked up by Tracker and the police (the cars with four identical antenna on a square base about 18" apart, usually mounted under the light bar on the roof) and this uses advanced direction finding techniques. The more sophisticated ones use GPS and mobile phone technology to relay their location, speed and direction. In other words, your insurance will know when you have been and when you were speeding, but on the good side, they will know where the car is if it is stolen or crashed into a ditch and you can't call anybody. The company, Tracker, have a lot of info on their website.