The most useful feature in S1 for me is quantize in realtime. After you have recorded your midi, you can loop playback and play with the quantize setting and hear it in realtime. Can't do that in Sonar.
The biggest draw for Sonar users to convert to is it's ARA melodyne implimentation. It is at least as good as Sonar's.
The keyboard shortcut might take a while to learn but it was well worth my time.
Comping was a breeze.
The only thing I miss was Prochannel.
I have been a Sonar user since 8.5 and S1 for 2 months.
S1's project page was very well designed and great for mastering and putting a CD project togather.
S1 might not feel sexier than Sonar but it get gets the job done faster.
Last but not least, due to it's larger user base, there are a lot more youtube instruction video compared to Sonar. Even in groove3 there are a least 3 times moreS1 videos.
I love Sonar but I believe in being versatile.