Qstarz BT-Q1300S Bluetooth GPS Sports Track Recorder with Arm Strap

Pros: So light you can barely tell you're holding it (or that it's clipped to your hiking pack) and super small - it's about 3/4 the size of a credit card.
Super simple one-button operation - hold for 4 sec to turn on/off, hold for 2 sec to switch between logger/bluetooth modes, click to create waypoint.
Software is free and available from the manufacturer website.
Very long battery life - I haven't actually run it out yet, even after 12 hours of continuous use.
Bluetooth connectivity works just as advertised.

Cons: Not as accurate as SiRFstar III chipset GPS devices (I tested it heavily alongside my Magellan Triton 400).
Performs poorly when shaken vigorously, like clipped onto a pack while you're running - but works fine when worn on your arm with the included arm strap.
Device implements a USB-to-serial adapter with the NMEA 0183 standard, not the USB mass storage protocol. This means that to extract track logs, you must use the supplied software which is Windows only - no drag-and-drop, no Mac compatibility.
Software is a tad clunky and overly crammed with weird pointless features. Works fine for: extract data, export to kmz, import into Google Earth. Otherwise simple.

Other: Fantastic GPS track logger. Definitely recommend.

Pros: I tested this thing extensively side-by-side with my Magellan Triton 400 GPS (with SiRF Star III chipset) and found the accuracy to be very similar - though it doesn't perform well if you shake it while running (try to attach it somewhere on your body that doesn't move around too much - like your upper arm).
Battery life is easily 10 hours, if not 12 when only using it as a logger (i.e. bluetooth turned off).
Operation is by ONE button, and super simple, though deciphering the Engrish instructions may take a few goes.
Weighs so little it's hard to tell it's in your hand.
Supplied software is rudimentary, but does the job.

Cons: Biggest con is that the USB interface doesn't implement the Mass Storage protocol, so you have to use the supplied software to extract your track log as a GPX or KML file using a USB->serial interface, so you can forget Mac compatibility.
Other than that, this thing is great for bushwalking or sports where you're not allowed a GPS with a screen (like rogaining or orienteering).

Other: If you want a lightweight GPS track logger that is accurate to 2-5m, buy this thing.
If you want a hand held GPS with a screen, buy a Magellan Triton - you can find the 400s and 500s for about $130, and once you get Mobile Atlas Creator, you basically have free maps for it.