I've started to pick plum tomatoes from one of the plants grow from a supermarket tomato and they've been very good. The peppers have set more fruit that the ones I grew from 'proper' seed but they're still too small to eat so I'll have to wait and see. I've often got butternuts growing from bought ones but found that they never got going fast enough for the fruit to ripen properly. This year I've got two plants going from the organic catalogue and I've already got a large butternut on one, so I think I'll stick to bought seeds for them in future.