Just bought a set of 16 Easi-Speak microphones after reading a thread on the TES website. They cost about £23 each if you buy in bulk. The microphone allows pupils to record their voice and play it back either from the in-built speaker or through a computer. The recorded files are in MP3 format and the microphone has Audacity built in, so when you plug the mike in with the attached USB plug, your computer immediately picks up the files and you can organise them. The quality of the sound is good if you speak close to the mike, but a little fainter at distance, so I wouldn't recommend it for oral exams where you need to record two voices at distance. Build quality seems fine and they are a doddle to use. I plan to get students to use them in pair work in the computer room, so they can hear themselves back on headphones with good quality sound.
Here's an aexample of a lesson using the microphones with a Voki.