I got S1 as a backup when the news came. Never switched though.
I've recorded and mixed in it, as well as REAPER and (mixed in) MixBus.
I know SONAR much better, but had no issues working in either DAW's.
I work in Logic at times as well (at a friends studio), and see them all as tools.
I just know some tools better than others

bluzdog

abacab
If PreSonus would add a chord track and an internal notation view, then it might become the "One".  It does just about everything else well.

The Notion integration is pretty cool! It's an add-on but it's worth it especially if you catch it on sale.
 
Rocky

I know, I have Notion, and basically stole it for a crossgrade price + a discount deal. 
 
It's really nice!  But there is still no notation view directly in the DAW.  It would be nice to just flip on a notation view when a MIDI clip is playing back in the DAW.  Editing the MIDI in a DAW PRV and viewing the notation in the DAW at the same time is useful.  Even Reaper can do this now.  Sonar has had it for ages...
 
And you may notice some griping over at the PreSonus forums about the need for a fully integrated Notion notation view in the DAW... 

WHAT I LIKE ABOUT S1P VER 3.5:
 
1. project page. worth the price of admission alone.
2. IR maker. same as #1.
3. Pro EQ- very straightforward, waterfall, lots of visual info.
4. bus routing: the way it should be.
5. VCA: pretty cool for setting up automation without going to the bus.
6. ampire: sounds as good as anything else i've heard.
7. groove delay: boy is this fun.
8. multiband dynamics: could almost replace waves LPMBC.... almost.
9. Pipeline Mono/Stereo : whoa.
10. the meters. they are awesome.
11. automation: creation of curves is dead easy.
12. arranger track: way more powerful than what i had in sonar 6 pe.
13. scratch pads. whoa. don't know about these? well, i'm sorry......
14. browsing. the best of all platforms.
15. multi instrument panel, and macro control knobs. for some folks, this would be the main selling point.
16. stretching the faders in console view, all the way up to the top of the screen. whoa.
17. Macros. man, i haven't dug into this fully yet, but the idea that you can set up a deep array of multiple commands and have them all automated, in order, via a macro, is super sweet. the build process is dirt simple.
18. low latency monitoring: the way s1 does it, it's mighty powerful.
19. drag and drop functions: super easy to create sends by just dragging effects.
20. there's more than i have time for!
 

bluzdog
I'm diggin' S1 Pro. It takes some getting used to but it's pretty intuitive coming from Sonar. I picked up S1 on the crossgrade deal just to make sure I was up to speed in case Sonar became unusable. I'm happy having both Sonar and S1 as they have different strengths. Take it for a spin.
 
Rocky

That's pretty much my deal too.  +10 on knowing Sonar is a big help when it comes to learning S1.

I also went Studio One. I love the arranger view and scratch pad but my fav is the Remote app.  It works great on my ASUS ZenPad 3s 10. Besides wireless it adds a very usable touch screen to the mix. Really cool!

Small thing...
I'm not crazy about Studio One's "Clip Fade In/Out" (compared to Reaper or Samplitude)... as the fade curve choice is limited.  You can adjust the intensity of the fast of slow (log) curve (which is a step beyond SPLAT)... but it's "linear", "fast", or "slow" for the fade itself (no cosine option).