I've been using it for about 3 years now (the large ARM instance a bit less than that, but since about when it was introduced). There have been two outages about a minute each (seems like a no-no-downtime host migration, but I am not sure). For the cost, I am not complaining.
Disclaimer: I haven't tried this out myself. I found this blog post a few days ago when I first heard about the generous Oracle Cloud free tier.