Why? Well, they are forgiving, and accept neglect without sulking. They stay where they are put. They are cheap to run... no, those can't be the real reasons. "Think of their variety," exclaims Terry Hewitt, who keeps at least 10,000 of the things at his Holly Gate Cactus Nursery in West Sussex. If you are the collecting type, there are enough curiosities here for 100 lifetimes.
They are weird because they are survivors. In inhospitable areas - desert, mostly - they are often the last plants left after all other vegetation has given up trying. Their spines provide awesome protection against grazing beasts. In high, dry places where days of extreme heat are followed by freezing cold nights, they grow long, woolly wigs. The hair insulates the core of the plant and protects it against extremes of temperature.
And, like some very old married couples, different species living in the same place start to look like each other, because they have all been buffeted by the same environmental difficulties and have overcome the same obstacles. So how do you tell your cephalocereus from your espostoa, or your coryphantha from your gymnocalycium?
If you are me, you do not. I (shamefully) use the same blanket term, "cactus", to cover everything from the tall, angular pillar cacti of every cowboy film you ever saw, to unfriendly little pincushions that think they are working hard if they put on a millimetre in five years. It is a shocking admission.
To know them, you have to grow them, says Mr Hewitt. That is true of all plants, of course, but there is less excuse for not growing cacti, since any windowsill could accommodate at least 10 of them. Beginners' plants include the haworthias, but you would never guess the kinship between the members of this particular family. Some, such as H attenuata, are like gasping starfish; others (H truncata) look like neatly sawn-off stepping-stones.
Or you could try stenocactus, which eventually grows into a vicious little hedgehog about 5in across. But then it flowers, and suddenly, out of a tangle of spines that would be classed as dangerous weapons if they were mineral rather than vegetable, comes a neat, perfect posy of pink flowers.
This flowering is brilliantly schizophrenic, like a Hell's Angel seen powdering his nose. Reboutias (another beginners' plant) do it too. They grow wild in dry, mountainous areas from Argentina to Bolivia, and are classic cacti pincushions. They are rarely more than 3in in height, but in early spring they leap into improbable bloom with flowers of red, yellow and orange. Cactus flowers seem to have no neck or stem, but sit low among the spines, serenely outrageous.
The neatest flowers are those of the mammillarias, which make a perfect coloured circle round the top of each globe, a crown among thorns. This is another huge family good for beginners. In the wild, they are widely spread, growing in Mexico, Colombia, Guatemala and Honduras, but they all flower prolifically, even when very young. The first crop comes in spring, and it is often followed by a second and third flowering later on.
Many cacti are low-growing and so look their best in shallow containers. Mark Pedro de la Torre is one of the few potters in the country who make pots specifically with cacti in mind. He grows quite a few himself, displayed alongside showy succulents in the big bay window of his house in Herefordshire. Outside, sempervivums live happily in the same kind of pot. They are low and heavy with curved, doughnut rims.
The form is unusual and very kind to plants that are dumpy by nature. They are also strong, both physically (good red Staffordshire clay that won't flake or break in winter) and in the effect that they create. Garden centres have done cacti no favours by dressing them up in plastic sombreros and sunglasses. Mr de la Torre's pots restore a cactus's pride.
MARK PEDRO de la Torre's workshop is at The Courtyard, The Old Rectory, Stoke Lacy, Herefordshire HR7 4HH (01432 820500). From 4-6 December he will be at the Contemporary Crafts Fair, Queen Charlotte Hall, Parkshot Centre, Richmond, Surrey. The fair is open from 10am-6.30pm (but closes at 5.30pm on the last day). Admission pounds 4. His pots are also available from the Collection Gallery, Ledbury, Contemporary Ceramics, 7 Marshall Street, London W1 and Bourton House, Bourton on the Hill, Gloucs.
Terry Hewitt's nursery, Holly Gate, is at Billinghurst Road, Ashington, West Sussex RH20 3BA (01903 892930). It is open daily, 9am-5pm. Send two first-class stamps for a catalogue. A wide range of cacti and succulents is also available from Southfield Nurseries, Bourne Road, Morton, Nr Bourne, Lincs PE10 0RH (01778 570168). Open 10am-12.30pm and 1.30pm-4pm daily, by appointment only November to January. Send a first-class stamp for a catalogue.
For a comprehensive guide to the cactus family that is accessible and well illustrated, try The Complete Book of Cacti and Succulents by Terry Hewitt (Dorling Kindersley, pounds 15.99). Cacti and Succulents in Habitat by Ken Preston-Mafham (Cassell, pounds 10.99) gives a riveting account of what these plants can do when unconfined by pots and greenhouses.
The British Cactus and Succulent Society has more than 100 branches. For more information contact the Secretary at 15 Brentwood Crescent, York, North Yorkshire YO1 5HU (01904 410512).