We've used and loved the balance bikes. We tried tricycles too, but they didn't figure out the tricycles until well after they'd mastered the balance bike, at which point a trike was slow and boring.
From balance bikes, both kids progressed to riding a regular bike without training wheels around or before age 3.
My older kid started on the balance bike a little over 2, and my younger at around 20 months. It seemed like having a role model really helped.