Lovely other ½ bought me some 'smart goggles' for christmas, they project your speed and stroke rate on the inside of the right eye so you can get feedback as you go along - plus other stuff like head angle, heart rate and such. They work surprisingly well.
On the plus side, I'm already going at least 10 seconds quicker per 100m in a normal session in just 3 weeks of wearing them. On the downside, seeing your speed slowly dripping away as you get tired and bored, and having to work so much harder to hold it has stripped any joy out of just swimming. Hey ho.