I picked up a couple of the IKEA digital thermometers a few weeks ago but only got around to using them at the weekend.
I was sick of constantly replacing LCD stick on thermometers and the LCD ones only give a rough guide on temperature.
I had read about people taping the probe to the outside of the fermenter and I was a bit skeptical about this so I started by immersing the probe in the wort. I then duck taped the probe to the side of the fermenter and got the same reading (both 4C above ambient temp). The readout has a magnetic back which is handy for sticking the display. You can also set a temp alarm so if your wort reaches a certain temerature it starts to beep.
It was interesting to me that even at this time of year it's pretty tough to keep your peak-activity fermentation below 20C without temperature control. The room had a consistent ambient temperature of 14C (too cool to sit around in), yet the wort on both brews reached 19C. The beer brewed on Friday has now dropped back to 16C and the beer brewed on Sunday has dropped to 17C.