I got a personal trainer 7 months ago, and it's been one of the best decisions I've ever made. I find having someone besides myself to feel accountable to very motivating.
It's also nice to have someone walk you through all the movements, proper form, and telling you what to eat.
When I started with the personal trainer; I was skinny, lanky, nerd. In the last 7 months, I've added 25LBs of muscle. I'm still a lanky nerd, but I'm now a stronger lanky nerd!