I'm on my second dashcam. Traffic here is terrible, and the 'me first' attitude of some drivers made it a necessity if I ever have a collision to have video to back up my version of the events to the responding officer.
I bought a DOD LS430W dashcam for $169 CAD. It has GPS which I didn't want, but I could get this model cheaper than the non GPS model for some reason. It came with a suction cup mount, although I'm using the tape mount for permanent installation. (I found the suction cup would let go from time to time in the weather extremes.) I went with this one because the camera doesn't have that annoying fisheye view the cheaper cameras have, and the night view is the best I have found.
My earlier one took acceptable video during the day, but you couldn't see anything at night. With this one, if you can see it the camera can capture it. The resolution is good enough to read licence plates at distance, and the colour reproduction is excellent.
To answer your questions, dashcams use an 'endless loop' to capture video where essentially the new video erases the old video as you drive. The amount of video you can capture at one time depends on the storage capacity of the card you get. My card is 16 meg card which allows me to store about 3 hours of video. If you want to keep something, most models have a button that saves the part you want and skips over that part when re-recording. An easier option is simply to unplug it or remove the SD card.
My previous dashcam had a split screen which allowed my to capture front facing and rear facing video in my truck. Although this would seem to allow for a back up cam, the display screen on most units is only a few inches square, and if you have both cameras running the screen is split between the two views.
I avoided the mirror replacement style because the better ones were more expensive and my OEM mirror has things built in to it that I would lose if I removed it.
Tips? Get a fast SD memory card. The video is particular about that, and there is a lot of information being transferred quickly. It's not really necessary to get a large card as the info recycles itself.
Turn off the GPS. It can accurately calculate speed.
Turn off the audio. I don't want anyone hearing my singing or after event commentary.
because it's a little more unobtrusive. A concern might be operating temp as it gets cold here. My truck has On Star which I've never used but the mirror post attaches to the window frame as opposed to a windshield mount. So the mirror/camera would be supported better.
