Edit: Also, I missed a good overview of features in one language and lacking in the other. In that respect, I find the wikipedia page [1] deeply broken, but that aside.
[1] http://en.wikipedia.org/wiki/Comparison_of_programming_langu...
What's not stable is everything around the core ideas. Many things are in flight at the moment in anticipation of 1.0 stability. The core collection libraries had a major refactor land last night. How error handling works got changed earlier this week.
If you're interested in writing libraries for a new ecosystem, now is the time to get into the language. I've tried to get into Rust a couple times since the 0.4 timeframe and I couldn't keep up with the pace of change. My current attempt started at 0.11 and while there are steady chanes, I've found them to be manageable.
If you want to have idioms and ecosystem relatively settled, hold off until the 1.2 or 1.3 timeframe (6 week cycles). I say this because a lot of the current RFCs are split between things that need to happen before 1.0 and things that need to happen after. This generally means the base functionality in-place for 1.0 with the sugar/convenience stuff to come later. I expect it'll take a few cycles after 1.0 before Rust settles into what most people are thinking when they think stable.
Huh? I'm seeing comments in this thread about people using Rust right now in production. How... why... what?!?
We don't recommend it currently, but some people are just eager. :)
Nope. That will take many years. But it definitely is in a state where you can start building real things with it, if you don't mind some of the usual costs of being on the bleeding edge, like documentation that's hard to grok, frequent trips to irc, and frequent breaking changes. But breaking changes will be going away very soon, the documentation gets better all the time, and the irc room is far friendlier than others.
Trying to figure out new things tends to be an exercise in reading obsolete code examples and then asking for someone in #rust on irc.mozilla.org to translate. Stuff that gets deprecated isn't well documented in my experience and one of the best resources I came across (Rust by Example) also contains a lot of out of date or deprecated features.
Obviously, this is a side-effect of a rapidly evolving language that is still in pre-1.0 phase, but anyone going into it should be aware of the potential breakage that tends to happen at a fairly high clip at the moment.