i came from a line up of i4 bikes, the last being an r6, before getting on a ducati so i'll give you my
i'll start off by mentioning that i didn't jump on the standard 748 so my comparison may not apply.
is by no means down on power compared to any 600 and up big on torque. however, it's also up on weight by about 40+lbs. you are correct regarding the standard 748's as they are down on power, but the torque is still there. the power or lack of power has to do with head design or efficiency and evolution/technology favors the i4's in this department.
the big difference is power delivery. i4 bikes are dogs until 7k rpms and pull like someone pushed a nitrous button from 9k to 14k. because of this they require a much more careful application of throttle...especially mid corner or exiting. twins have a very broad powerband ussually starting as low as 3k rpms that pulls EVENLY all the way through to redline. this makes getting on the power easier and therefor possibly earlier when exiting corners.
besides the even powerband and torque, the advantages the duc's will have over the 600's is corner stability and feedback. out of the box, you will never get a japanese bike to out perform a duc in these areas.
service intervals are the same (when going by the book), but valve maint is where you hear the most hype. the valve train on ducs does require a more proactive approach. you can't miss a check up like you can on a japanese bike and not risk big money repairs. there are aftermarket valve retaining systems that will basically give you the same reliability and valve service interval as any japanese bike if you decide to go that route. finding a good mechanic is the first step and most ducati owners know NOT to go to a dealer (although there are a few good ones) and because of this i have found service costs to be no greater than what i paid for the r6.
in my experience, overall reliability is no less than any of the japenese bikes i've owned.
exclusivity is not the main reason i bought a duc, although it is one of them. it's nice to go for a ride and not see EVERYONE on "your" bike. the passion i have for this bike can't be compared to any other bike i've owned. the japanese have been trying to copy the looks of the "superbike" for years...without success. the japanese do make some good looking bikes but they still pale in comparison when side-by-side to the italian version. and the sound of the ducati twin is again unrivaled by ANY i4.