I personally have friends who will trim their own horses feet for maintenance, but the majority of their horses are pasture puffs and lawn ornaments. In their case, they are saving money by not having to call a farrier regularly but they do have their feet hooves professionally done a few times a year to ensure there are no issues.
For performance horses, I would leave the trims to the farrier unless the skill has been studied intensively and they are confident. In Ontario we have the Ontario Farrier's Association (http://www.ontariofarriers.com
) and a lot of the registered farrier's offer apprenticeship programs, which I think is fantastic for anyone interested in learn the skill. :)