One of the best effects I find in metta practice is it's ability to improve my anapanasati. The fact of the matter is, until one achieves non-returnership, one will not have eliminated hatred. When anger takes over, it is often hard to apply our wisdom. I guess what I'm saying is you shouldn't rely on widom to always be there for you. Practicing metta ensures anger will not arise as often, or have such a controlling effect.
I wholeheartedly think everyone should practice it. While it's not a necessity for achieving liberation, it has so many benefits, and produces an astounding amount of good kamma. I'm trying to find the sutta, but in essence the Buddha lists a long series of things you can do to accrue merit, including giving tonnes of gold and silver away - I seem to recall the number 100 being thrown about... and anyway he says practicing metta for just a short time will yield more merit than that. I believe the sutta was in the Samyutta Nikaya or perhaps the Anguttara. I'd love to find it again if anyone knows the one I'm talking about.
Modern science is coming out in favour of metta practice, from wikipedia:
A few recent psychological studies suggest that loving-kindness meditation may impact health and well-being. One study done at Stanford University suggests that a short 7 minute practice of loving-kindness meditation can increase social connectedness. Loving-kindness meditation has also been shown to reduce pain and anger in people with chronic lower back pain. Researcher Barbara Fredrickson at the University of North Carolina at Chapel Hill found that loving-kindness meditation can help boost positive emotions and well-being in life, fostering the personal resources that come from experiencing positive emotion. More research is needed to see whether loving-kindness meditation is appropriate for all populations, whether it works similarly for everyone, and to understand how much practice is needed for the benefits of the practice to manifest.
An EEG study by Richard J. Davidson of people who meditate in metta, with a minimum of 10,000 hours practice, showed substantial differences in the magnitude of gamma waves as well as gamma synchronization, particularly during meditative sessions, and directly afterwards. During baseline states, where the subject was not engaged in the practice of metta, there was a signature brain wave pattern that distinguishes the metta practitioners, lay people as well as monks, from people, at baseline, who have not extensively practiced compassion meditation. This study also showed, during meditation, an increase in the activity of brain areas such as the temporoparietal junction, insula, and amygdala can increase the subject's ability to see things from another's perspective, and actually change the area of the brain that is involved with the autonomic system so that the meditator's heartbeat increases. These studies show that the amygdala is modulated during compassion meditation. Compassion meditation has been shown to lower the participants reaction to inflammation and distress, both of which are associated with, "major depression, heart disease and diabetes," in response to stressors, a change that was dependent on the amount of time spent practicing, with practitioners who spent more time meditating having corresponding more significant changes in their brains.
The Vissudhimagga lists a series of types of personalities a yogi may have, and I am of the variety where hatred is the dominant hinderance. So I find metta very beneficial. All the same, even people who are not inclined towards anger and hatred will see manifold benefits.
You say that sometimes you have negative thoughts that you just can't stop, that are like a runaway train. This is where I think a regular and established metta practice would help
It only took me a few days to notice the very welcome effects it had on my overall mood and my relations with friends and family. It's all positive
Kind regards & metta