The 675 is considerably narrower than any other 600 on the market currently by virtue of the frame design and the narrower 3 cylinder engine.
The torque curve is where the major difference lies between the 675 and all the I4's.
All modern sportbikes are excellent pieces of machinery, designed to perform well above the skill level of most people who buy them (myself included) and it boils down to what you are going to use the bike for.
The I4's don't really come into their power band until you are revving the ***** out of the motor while the 675 pulls hard from fairly low in the rev range. Thus, making it a more streetable supersport and also giving you more drive coming out of corners on the track. The I4's shine on the track when you can ring the ***** out of the motor all the time.
The other reason I like it so much is that I want to be different from everyone else and there are far fewer 675's on the road than Kawasakis, Suzukis, Yamahas and Hondas. Plus in my opinion, which is really the only one that matters when I'm buying something for myself, it is way better looking than any other supersport bike on the market. I may change my mind when MV comes out with their 600, but for now, it is the best looking one out there!
Find a shop with a demo and take it for a ride. But before you do, make sure you have your money ready becasue when you get back from the test ride you will buy it!