I had never tried one and saw them in the shops the other day.  They are similar to a passionfruit and a tomato.  Related to tomatos, eggplants and potatoes they orginated in South and Central America.  The skin is not edible as it is quite bitter but the seeds and membrane inside is.  The one I had was a bit more bitter than a passionfruit.  They are high in vitamin C, fibre, Vitamin A, and Vitamin B6.  They have a good level of potassium in contrast to sodium which is beneficial particularly for those with high blood pressure.  Contains the trace elements copper and manganese.  Even though they appear more like a fruit, apparently you can cook with them as you would a tomato.  Have a try and see what you think!