My 2p.
There are 3 around, original, "expensive" clone and "cheap clone".
Original, around £70+ these days, 25fps, metal body with slider switches.
Expensive clone, around £30-40 (
Maplin sell this one... at around 60 I think...), 20 or 25fps (hit and miss), metal body with sliders switches.
Cheap clone, around £10 from china all in, "30fps" but a lot of people have reported this to be 15fps with duplicate frames to fake the 30fps, plastic body with push button switches and are known to freeze randomly (hence a reset button which isn't present on the others).
Batteries are not "user" replaceable, but are hackable, has a 350mA 3.7 single cell LIPO.
None of the units can record whilst charging.
All of the units cut every 30 minutes - this is not because they are cheap, it's common on most units to cut them up (because they're recording with headers, so if you lost power you lose the section).
If you lose power, it doesn't write the header of the video file, so you lose that section.
The time in between segments is mostly below 1 second - sounds bad but is much better than the more expensive "MiniDVR2" (or angeleye) which has a 4 second gap, and I think the ContourHD has something like a 2 second break too.
Oh - adn when you get the Micro-SD card, make sure you get a class 6 one (fast). Otherwise the video capture could be too quick for the card to handle.
You don't need a very fast one, just a "not crap" one. Which is usually the problem if its one supplied "unbranded".
Class 6 is overkill, 4 will suffice - even then I have used some Class 2 perfectly fine (as long as they are REALLY Class 2).