Gristmill has an interesting article on genetically modified plants. In summary, Jeremy Carl reports that GM crops reduce pesticide use, reduce the demand for new lands by increasing productivity and by being able to grow on some degraded soils, and increase the income of poor farmers.
The article also comments the objections put forward by some environmentalists - that GM crops may be bad for our health, that they result in the loss of traditional plant varieties, that it is unnatural to put genes of one species into another, and that it is wrong "ADM or Monsanto making choices about what [we] eat."
GM crops have been so far harmless to humans. Replacing plant varieties with new ones that are more productive is something that farmers have done since the origin of agriculture. Putting genes of one species into another is a very natural thing that happens all the time without human intervention (for example through viruses and plasmids). And consumers have a wider variety of choices thanks to GM crops (but, of course, there are people who complain of too many choices).