My mid-2011 MacBook Air with 4 GB memory has been running really well. Occasionally Safari (the Metal version in High Sierra) gets slow, but it's still faster than Chrome. I've been using it for general casual computing and machine learning development with Jupyter Notebooks and haven't run into anything that would make me want to switch. I'll probably upgrade to a 2018 MacBook Pro with 8 GB memory when those come out.