Hi, it depends which Test n Tag model you are considering.
The Test n Tag Printer needs mains power for it to work, the 90 x 25mm labels are very durable but expensive. Depending where you buy your cartridges a single label can cost from 16 to 32 pence. I've had trouble with the ribbon breaking but you can open up the case to repair it. Not something you want to be doing in the middle of a batch of tests though!
The Test n Tag Elite model is rechargeable battery powered. Labels are 75 x 52mm, but fade in direct sunlight over a period of time. They come with a central horizontal perforation which is a weak point when applying it as a cable wrap. However you can print on half the label, split it along the perforation and then re-apply the blank half to the roll, giving you a cost per label of approx. 5 pence. (Size then is 37.5 x 52mm)