There are quite a few options out there - so many that it almost seems overwhelming. About 3 months ago, after watching a number of "idiot driver dash cam" videos on YouTube, and thinking about the fact that I drive 110 miles round trip for work every day, I started looking. I considered quite a few, from the cheapies, to some that were close to $100 each. I also was considering between those that mount 'permanently' to the windshield with adhesive, and those that mounted with suction cups.
I did a ton of reading, watching review videos, etc.. and narrowed it down between one that was very highly rated just about everywhere, had some excellent features, and mounted 'permanently' to the windshield using adhesive, for about $70... and one that was between $20 and $30 (depending on where you buy it), mounted with a suction cup, and had the "essential" features.
... ultimately, I decided on the cheap one, for a couple reasons:
1 - In Phoenix, our windshields tend to not last that long. Between "sand blasting" on the highway, all the trucks that kick up rocks, etc.. and the heat, it's not uncommon to replace a windshield every year or two. Mounting a "permanent" dash cam to a windshield that is going to be replaced frequently, to me, just makes for a hassle when it comes to re-mounting said dash cam.
2 - I wanted to do a 'test-run' on a dash cam, partly to see how much of a visual interference it was to my field of view, whether the Phoenix heat will kill it (it's supposed to hit 122 degrees today, which means potentially 150-160 INSIDE the car in direct sunlight), how much of a pain it is to run power, etc. In summer here, stuff just COOKS inside any car left in the sun, and small, inexpensive electronics, especially those with batteries, are especially susceptible to this.
So, I made my choice, and purchased a high-quality 16GB Class-10 MicroSD card for storage, and started using it.
I purchased This Dash Cam on Amazon
If you search on Amazon, eBay, Newegg, etc.. you will find tons of other listings, under many different names, for the same thing. I've seen it as low as $13 in a few places, plus shipping. It's a cheap-o unit from China, but surprisingly enough, it does the job.
If you search Google for "GT300 dash cam" you will find all sorts of info, links, etc..
So far, I have to say that, while it is far from perfect, and does have its shortcomings, it functions exactly as required, and seems to record in pretty decent quality. It records video in 3-minute chunks, storing them to the SD card. Once the card fills up, it deletes the oldest file and starts a new one.. so basically an endless loop of recording. There is a shock sensor in it, which will lock the currently-recorded file if an impact is sensed, and of course, you can manually choose to lock the currently-recording video if there's something you want to keep. The cam will not delete any files that are marked as locked.
The unit I received has since developed a dead-spot on the screen - My guess is this happened because I accidentally left the cam on for a full day, while the car sat in my driveway, and some stationary glare burned out that part of the screen. This has not affected the recorded video, however - just the screen itself.
The cam comes with a rather long cord that ends with a cigarette lighter attachment. I have the dash cam mounted near the top of my windshield, behind the driver-side visor, and the cable is long enough to run along the headliner, to the driver side A-pillar, down to the floor, under the floor mat, and to the power point in the center bottom of my dash. I will be changing this out with a more permanent power solution soon, as I am ordering a new power lead and a fuse block adapter, so the cam can run off of switched 'key-on' power.
As I mentioned previously - It's getting HOT here. So far, we've had at least one day over 120 degrees (ambient), and multiple days over 115 degrees, as well as dozens over 100 degrees, and my car has spent quite a few of these days parked in the sun, with the dash cam mounted in the car. So far, aside from the issue with the dead spot on the screen, the camera doesn't appear to have suffered from this heat.. Since it's only $20, I'm sure that at some point, it will probably succumb to the heat.. but I suspect it will make it through this summer still working.
Hope this helps.