I have a 58 reg 2.2 Ti, which I bought with 100K on the clock. I was wary of the mileage but the price was very reasonable and the car came with a full history. The main issues I researched were timing chain stretch, steering rack replacement (wrong power steering fluid from the factory), and gearbox whine. Mine has never had a replacement chain (and my Alfa indy does not recommend one), it has the wrong p/s fluid but the steering rack is fine, and there is no sign of the gearbox whine.
The main downside specific to the engine is the hesitation issue. In stop/start traffic, it is a PITA. Otherwise, I have to say I like the engine. It is eager, responsive, and gives nice linear power delivery. It is a lovely car to drive on the open road with good handling. Sometimes when I read the forum, I wonder whether I bought the same car as some other people ! Suffice to say, the 159 has problems like any other car, but not every 159 has the same problems.
Other downsides are MPG, and ride quality. The 19" alloys give the car a firm ride, and coming from a 166, there is a marked difference in comfort and ride quality which someone coming from a non-exec car may not notice so much. FWIW, I would recommend the 2.2. If you want a bit more comfort, check out the Lusso, but it's very hard not to get enticed by the looks of the Ti.