I have one presently and am moving to an R1 !
The 675 is a fantastic bike. The triple is a lovely engine... the stock exhaust is plenty loud and sounds good stock unlike quite a few other manufacturers. I wouldn't say it sounds 'better' than the r1.. the r1 is deeper and has a lovely wail when you get it going.. the triple has a snarl and a metalic rasp. As for power ... to be honest both bikes would be more than ample for me. I wouldnt tap either to anywhere near their limits. Where i live the roads are not great and you ride to the conditions. Ive done the advanced rider training on both (every time i buy a new bike i do this .... its a great way to get accustomised to a bike). With the new r1 im getting, ive no intention of getting rid of the chicken strips (due to fact it pisses rain 99% of time here and roads are unpredictable here). Il wait til i get competant enough on the track for that

.
Why am i changing ?
Comfort. I use the 675 as an every day bike.. doing small commutes to and from work ~ 8 miles or so. Then at weekends i do trips into the country side with mates ( 2 to 5 hours trips on average). I'm 6'2" almost 6'3" and 190lbs. The 675 is more like a 400gp bike than a 600 in terms of size (this was originally why i bought it... a decision of the heart and not the head). Beautiful and skinny! Italian styling without italian reliability.
Build quality is fantastic. I haven't had it long enough to question the reliability.
My only and main gripe is that when i would finish riding i would often find my back sore and wrists not too comfortable. The seat height is absormally tall.. (it doesn't affect me reachign the ground... ive long legs). But it does throw a lot of weight over the front bars.... more like the R6 does than the r1.
Ive taken a loan of a mates gsxr and blade already for a day. They feel like sitting on an armchair to me after the 675.
That is not to say im tryign to put you off the 675. Its a magic bike. It looks gorgeous (nicer than a 1098 to my eyes but beaty is in the eye of the beholder), has a phenominal engine for a 600 class bike, and is a brilliant track day weapon). Just if your bro wants a 600 class bike for do it all.. i reckon the new 600rr is the way to go. Its just that bit more comfortable than the 675. If i was doing it all again and coudlnt afford an r1.. i would go for the new 600rr.
P.S. one other problem you may find ... and its something that pissed me off a bit. Its harder to get aftermarket parts for the 675. That said if your bro is looking for after market pipe... tell him to look for a 'jardine'. BEAST of a can. Stunning sound.
http://www.youtube.com/watch?v=2UeZC8pt-ac