In reply to Denni:
I looked at the Transcend and decided it was a bit pricey for the spec. It's also quite bulky, and the screen could be distracting - I believe that might be a problem if the plod suspect that you might have been staring at it rather than watching the road...
I have one of these:
https://dashcamtalk.com/mini-0801/ with the GPS mount. It's quite compact and, in my car anyway, can be mounted on the windscreen behind the rear view mirror so it can't be seen when driving. I have it hard wired in to an ignition live circuit in the fusebox, so it comes on automatically when I'm driving and turns off when I'm not. I use Samsung 32GB Class 6 micro SD cards in it, which give me about 5 hours continuous HD recording (in seamless 3-minute chunks). It doesn't need Class 10, and some users have reported problems with some Class 10 cards.
I tried the 0803 which is a newer version of the same device with higher video resolution but I didn't get on with the configuration options offered by the firmware at the time, and I decided that the higher video resolution wasn't worth the aggro.
That dashcamtalk web site is good for reviews of other dash cams. It also has fairly active forums where you can get help and advice and dash cams in general, and specific models in particular. Thanks to that site I was able to turn off the annoying sounds that the 0801 makes when it powers on and off. Another good web site for dash cam reviews is
http://www.techmoan.com/blog/category/car-dvr-dashcams
You do need to be a bit careful where you buy the 0801 as there appear to be some poorer-spec clones out there. The dashcamtalk forums can be a useful source of information on reputable and less trustworthy vendors. IIRC I bought my from Amazon - fulfilled by them rather than through a marketplace trader so it arrived in days rather than weeks on the slow boat from China, and it's easy to return.
If you (or your insurance company) need a camera that will also protect your car when it's parked, with things like motion detection and IR illumination, then you will need to spend quite a lot more. A lot of the cheap cameras claim to have this kind of feature but they typically don't work well enough to protect the car if its parked up for an extended period. You also have to keep the thing powered on all the time (although their consumption is fairly minimal).
If I can find the time to do it I'll post up a video clip from my dashcam of the idiot in the white BMW who nearly took me out on the A701 the other weekend. It's that sort of incident where a dash cam could end up being useful for proving who was at fault.