Our daughter, at 2.5, loved it! The video of her squealing with delight is a favorite from that trip. We ended up riding multiple times in a row.
My daughter is very sensitive and cautious, but I also know that she loves many things that she is initially afraid of (e.g. tall slides, power wheels, etc). We took her to a local county fair with carnival rides a few months before Disney. One ride was a mini-coaster (a short oval track with two hills - slower and shorter than Barnstormer, but it would do as a test). She loved that ride, so I felt comfortable putting Barnstormer into the plan. I scheduled it as the last attraction before our break, just in case she didn't like it and would need time to regroup.