You can perfectly well save seed of F1's. You'll have a variable crop the second year, and if you pick your favourite to save seed from, and do this for several years, you'll have your own variety. Toms are inbreeders so it couldn't be easier!
It's great fun, saves all the hassle of making crosses too and skips the sometimes boring F1 phase. Someone doing it for te first time may be very surprised at the results, you can get all manner of colours and shapes of tomato from seed saved from an F1 red globe cherry.
It's also worth growing seed from shop bought toms, we had a delicious grape sized plum tomato from lidl, we saved seed and sowed this year, so far there has been no visible segregation, but maybe some will become apparent when the fruit ripens.
However, many so called F1s produce no segregation at all, seems those seed companies are prone to lying just to justify their high prices and to discourage us saving the seed.
