Very happy with my first real try with Reaper

Tonight, I started my new song. It should (eventually) feature acoustic guitar and voice, and include a couple of other instruments in the background. After much deliberation, I started off in Sonar, because I'm so much more familiar with it, and because ARA is still in Reaper's future. I also figured that I could use some more time to absorb the Reaper videos. Well, it's a tricky song, as far as tempo changes go. The different sections flow into each other, but aren't the exact same speed. I started by trying to play the acoustic guitar to a click for the many changing tempos I had figured out in advance. (I need the click because a couple of quiet MIDI instruments will be added later, and they are tempo-dependent for their effects.) Unfortunately, the guitar ended up sounding a little unnatural, a tiny bit hesitant in one transition and a little hurried in another as I tried to hit the new tempos just right on the fly. I'd normally just start again. But in this case, I had (amazingly for me!) made it through the whole four-minute song in one take without messing up any of the notes or dynamic feel of any of the long technical passages. It's a flowing song, and the guitar is completely exposed, with notes ringing throughout. You can't really chop it off at any point to insert a new take. I really wasn't sure my nerves could take trying to accomplish playing through the whole song that well again. So I exported the track to Reaper and used time stretching to just tickle those two passages so they felt just right. Hurray for Reaper! I could never have done that in Sonar. As soon as ARA arrives, I'll use Reaper exclusively. Thus ends the lesson of the first try. I'm really pleased! :)

What do you use ARA for, I'm not sure I have a grasp on what it does?

Per Wikipedia, "With ARA, the DAW and plug-in can exchange information about the audio file, tempo, pitch, and rhythm, not only in the moment of playback but for the whole song. The transfer of audio material to Melodyne isn't required anymore." I still use Melodyne Studio in conjunction with ARA because it makes everything that much simpler and obvious visually. Briefly, I use it to fix imperfections and other annoyances that otherwise wouldn't be fixable without much hair-pulling, if fixable at all. No opening outside processes, doing work and transferring back and forth. It's all done from within the main DAW. Drag your free playing to the time line and ARA figures out the whole tempo map that can be followed by your MIDI instruments, for example. It makes editing quick and seamless instead of a pain in the neck, at least for me. I know this isn't a great technical answer to your question, but I hope I gave you the general idea of what it does.