Depends on what you mean by solid. It uses LLVM for all the codegen and perf is generally in line with C++. The core ideas around traits/borrowing/inference/mutablility have been stable for a while.

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.