Companion planting - is it rubbish or is there something to it?

Here's a list of the things I've been told to plant together or seen planted together:
• French marigolds in between tomato plants
• Carrots and leeks together because they have strong scents that drive away each other's pests
• Nasturtiums with cabbages because the nasturtium leaves are apparently more popular with caterpillars than cabbage leaves (I'd love this to be true!)
• Garlic planted among roses to ward off aphids

You see, I'm about to order my seeds for next year, and I was wondering about all this - I can bring home some garlic to put in around my roses, and I can harvest nasturtium seeds from any of my neighbouring allotments, so should I get some French Marigold seeds and try the experiment? Or is it complete tosh?

Labels: allotment-companion-planting, allotment-seeds