Well, I'm in no way a Reason expert but, there is most definitely something about it that makes everyone flip out. Once I learn that, I'm sure I'll be kicking myself in the ass for not diving into it sooner, but from what I can tell it has some pretty amazing features. The whole process of routing and all that exists because you can quite literally set up the rigs to do pretty much any damn thing that you could think of doing on a physical setup, as well as some features that can only be done with a computer. Now although I can patch well, I don't find it necessary for me to do so at the moment, so I've held off on it.
As for using Reason along with Ableton, there should be absolutely no problem. It works like most other DAWs in the time sync aspect, the benefit is the ability to expand on your main DAW and incorporate the special abilities from Reason (or whatever other program you're running).
Now the Pros and Cons of Rewiring vary.
Pros:
No physical space taken up.
No need to worry about syncing with an external device.
Cons:
May eat up more cpu than you're comfortable with.
To control parameters within the Rewired program externally, you may have to sacrifice control parameters on your external device.
Unless you have a method that you're comfortable with switching windows, the extra space that the program takes up on the screen may be a bother (But simply Alt+Tabbing should resolve this easily. I'm sure there's even a way to program a device to alt+tab for you with the push of a button.)
Now I'm sure the list continues, add onto it if you'd like. But these are just the main elements you find when Rewiring.
