On a pretty large rails app, we switched to 1.9.2 about a year ago. Startup time for our app is much, much worse than it was under ree, but our average response time significantly improved -- we saw close to a 50% drop in the time spent in ruby code across nearly all requests. I wouldn't use slow startup time as a reason to avoid benchmarking your app in production mode against 1.9.x, since it could result in a huge perf boost across the whole app.

There's also a reduction in the memory footprint in switching from ree to 1.9.x. I can't seem to find the article detailing memory usage, but there's a decent difference.