My advice is that grafted plants are worthwhile if you are growing tomatoes in the same soil year after year (usually in a greenhouse).
You are unlikely to see a benefit from the disease-resistant rootstock if you are growing in grow-bags or containers using fresh growing media. You may still benefit from the extra vigour resulting in an earlier first pick or an extended harvest period at the end of the season. If you buy grafted seedlings, make sure the graft is above the surface of the compost when you pot them up or plant them. If you don’t, the scion may produce adventitious roots and you will lose the benefit of the rootstock.
Commercially, tomatoes are grafted at the seedling stage, but this is very difficult to do successfully at home. It is possible to graft bigger plants – look in old text books for instructions e.g. The UK Tomato Manual (1973, H.G. Kingham (ed.), ISBN 0 901361 14 3).
Did You Know?
British tomato production amounts to about 75,000 metric tonnes per year - about a fifth of the total volume of tomatoes sold in the country through the year, and up to a half in the summer.