I had an ST3 for three years and loved it.
Good points:
Performance:- the three valve engine gives tons of torque and a good top end, the pull is strong all the way through the rev range. If you are going to do a lot of urban driving, it's a good idea to change the standard 15-tooth front sprocket for a 14-tooth one to make the engine feel more flexible. The extra acceleration doesn't hurt either!
Handling:- as with all Ducatis you'll want to get the suspension adjusted to suit you. Once you've done that, you won't believe how chuckable a sports tourer can be! The ST4 has higher quality suspension components with more adjustment options, its engine is identical to the ST3's. I never drove an ST4 but my ST3 had a superb balance of handling and comfort.
If you find an ST3/4 it will most likely have the standard Michelin Macadams fitted. In terms of handling these are by far the bike's weakest point and should be changed for a set of Pilot Powers or Diablos.
Build quality:- top notch, and yes, I know we're talking about a Ducati! Nothing ever fell off mine (not even me!) and the quality of the paintwork was up to showroom standard even after three years. I got one deep stone chip on the tank (came up from the road, hit my arm and bounced onto the top of the tank - bugger!) but even though it made a noticeable crater it still wasn't through to the undercoat, never mind the metal. It kind of looked as though the tank is powder coated. The electrics were thoroughly reliable, the only thing I ever had to replace was a dead battery. I guess I was just unlucky with that, and in any case it's not a Ducati component!
Wind protection:- as you've no doubt noticed it has a big fairing and a huge screen. The protection for me was so good (I'm six feet tall) that until I got used to the bike I regularly found on looking down at the speedo that I was doing 10-20 mph more than I thought I was.
Bad points
Although the seat is superbly comfortable, I found it a bit too soft after about 200 miles, because when you try to change your position to get some relief you just sink back into it and it feels exactly the same.
That's all I can think of just now, of course if you have any other questions I'll be happy to try to answer them.