The only technical problem I've seen that node is an easier solution is dealing with third party apis concurrently. It wouldn't be very straightforward to have a redis query, a mongo query, a facebook query, and a sendgrid query all open at the same time and managing the results when they all come in (and keeping program flow maintainable) in PHP, Python, Ruby, etc. Probably not a common problem or desired feature, though.
We're 100% Node on heroku with a custom buildpack, using a hosted Mongo service, some Node written cron jobs (which were pretty interesting to write), and redis and we serve 5 digits worth of hits every day at least, and I'm more than happy with performance. I never once look back and regret the decision to use node. I don't necessarily care about sharing code, having frontend developers do everything, etc like some node guys throw around naively, but I am looking forward to bringing somebody on that can grasp adding in a new API route, throw in a Mongoose find, and render a template with some variables. We have EJS templates serverside and clientside for that reason, and that could just as easily be done in PHP or Python.
However, I am enjoying working with node, the node community, and the interesting modules and stuff that are being released. I will say that there's a good bit more active "lets hack this up into a module and open source it over the weekend" kind of people in node than others at the moment.