I ride with a large cross section of guys, pretty much every modern make and model, and have sampled most of the them. Here is what I think...
Suzuki.... Stay away. There is a reason that suzuki's have a reputation of squid bikes... Becaue everyother suzuki rider has an idiot riding it with shorts, a teeshirt, and flipflops. There is nothing wrong with the bikes performance however, as they definetly on par with the other 600's (and the GSXR1000 its nasty). However, they are poorely made compared to others in the class and honestly my 3 of my friends gixxers (2 750's and 1 600) have spent more time in the shop (they are all 2005s) then the rest of our bikes put together.
Honda.. A CBR600RR is my current stead and I peronsally think its the best looking 600. Performance wise, its pretty much equal to all the other 600s (though the yammie puts out about 5 more horsepower at the rear tire). It is widely regarding as the best handling 600, and as far as realibility, lol its a Honda. The 4 600rrs in our group (2003, 3x2005's) have never had a problem. Not one....
Yammie.... Sweet bike that was supposed to break all the rule and revolutionize the 600 class. Unfortunatly a lot of the hype turned out to be just that, hype. The engine only revs to 16,000 despite the tach reading 17500 (which means you must sign a waiver when you by this bike stating that you understand that. The was a class action lawsuit against Yammie for false advertising). This bike does put of more hp then the rest of the 600 class, but you lose a lot of torque, meaning this is a great bike for the track, but on the street your gonna be twisting the throttle a little more to keep up. Still, a fresh, good looking design and light weight make this a good choice.
Ninja 636... I haven't rode the 636, but I will say on paper this should be the 600 king. Slipper clutch, good torque, good horsepower, claimed low weight. Only thing is, this bike constatly rates lowest on magazine tests.
Trimuph 675... More power, more torque, less weight, awesome styling, exculistivy, ect ect ect.... If I hand my Honda paid off, I would be riding this. Awesome 3 cylinder sound, faster then a Gixxer 750.. How could you not love it....
Anyways here is how Sport Rider ranked the 06 600's and 750's
1) Trimuph 676
2) CBR600RR
3) Gixxer 750
4) Gxxer 600
5) Yamaha R6
6) Ninja 636
7) Ninja 600