I agree with SMR's post regarding life with an SM450R - I've only had mine a few weeks, but I've really been enjoying it as my "commuter" bike, and it's terrific on the local "mountain" roads (at least the tighter ones). It's "harsher" than the KTM Dukes I've previously owned - no cush drive, harder suspension, (especially the rear, even with compression damping at its loosest setting), and the throttle really is sensitive.
On the other hand, it really depends on your expectations. I had a friend years ago who wanted to buy an '85 Kawasaki 900 Ninja after trying mine, after not riding street bikes for years, (although he had entered a few enduros), and I suggested that he might be a bit uncomfortable on what was considered to be an extreme sport bike at the time (he was pretty tall). He reminded me that he was racing bicycles at the time, so how uncomfortable could it be compared to that? He bought a 900, and loved it.
A couple of things to remember - there's no electric fan, horn, or headlight dimmer switch, and you'll definitely want to raise the gearing (I went to a 16T countershaft - stock is 15 - and I've ordered a 42T rear - stock is 45). The shop installed a "hidden" battery switch, so I carry a disc lock if I expect to park it anywhere I'd be worried about. I plan on changing the exhaust, but the stock pipe sounds ok and isn't too noisy. It's not a torquey thumper motor - you have to rev it a bit to really appreciate it, but it's a fun ride. It's also really small, for a bike of its type (much smaller than an LC4, I'd say). To me, that's part of the appeal, but bottom line the KTM would be a more comfortable bike. I'm glad I bought mine, though - I already had a Ducati, and I like the way the Italians build bikes. It's the coolest single I've seen, and depending on your own application you might really like it.