I use one everyday for many hours a day. Great machine. Long ago I took the RAM to 16GB and put in an SSD drive. Later I upped the original 240GB SSD I had at first to the Samsung 1TB SSD unit I have now. I've had no issues with feeling I need more speed. I have a quad core i7 running at 2.3GHz which was the lower end I think. It runs El Capitan without issue.
I bought mine in July 2013 and it is still under AppleCare until July 2016.
Just today Best Buy has a deal of the day for a 480GB PNY SSD for $99.99. Prices just keep coming down.
I think it is a good choice such that with upgrades that have been suggested in this thread, for under $800 you will have a very capable machine.