I use REW and honestly, it couldn't be easier or more robust.
The newest version, which is of course also free, now has support for Plug-N-Play USB Mics.
You can get an individually calibrated USB Mic from Cross Spectrum Labs for about $99 shipped and there is no pre-amp, phantom power, etc needed.
So there is only one piece of equipment to buy, the mic, and it's plug-n-play, the software is free, and as mentioned in a post above, you can take one measurement, one time, and then manipulate it any way you want to get any data out of it you want and until you learn (there are several threads that go into detail about how to read and interpret each chart/graph) you can also share the measurement file so more experienced folks can manipulate it and analyze it with standard graph settings that most of us use.
You can literally plug the mic in now, tell REW which USB Mic you're using, and take a measurement in less than 5 minutes for less than $100 with an individually calibrated mic.
IMHO it doesn't get any better than this and it's by far the cheapest and most robust solution.
Thank you for that - you are tempting me to try it if it is as easy as you say. I am happy with my OM but I am seduced by the idea of one measurement and then multiple graphs from it. I'd be more than happy to spring $100 for a mic just to try it out (I can always sell it if I don't get on with REW). Would you please suggest a mic, or mics, that I might want to try, which will work in the way you describe? Many thanks.