I asked the same basic question a little over a month ago, this is my reply a month after buying one
As a 147 owner you will be used to the 'quirkiness' of Alfas in terms of the style and reliability. The 159 series is the most reliable to date (from my observations).
Common moans from owners seem to be :-
1) Fuel consumption from diesels , the car will perform and drink fuel like a petrol. The economy of diesel from a 159 is only at the pump.
2) Tyre wear , some (but not all) 159s seem to have low tyre life. This is particularly apparent on 18"+ rims. Best known fix is to change the tracking to 0degrees. Personally I feel that for the grip and performance of these cars , 'poor' tyre life is expected.
3) EGR/DPF issues. The car is fitted with a load of 'gadgets' to clean up the emissions , EGR issues were only an issue on 06/07 models as far as I know. The DPF issue is more complex , basically if you are a 'stop/start' short distance driver that never does regular 50mph+ trips (for 10 mins plus once engine warmed) then modern diesels are not for you.
4) Windscreen creak. Rare but common enough to be heard of. I personally (with no evidence lol) believe this is due to owners washing their car with Fairy Liquid instead of proper car shampoo and drying out the windscreen rubber. It is known that a treatment of silicon lube (giggles) if you get this problem will cure it.
I bought a 2.4 JTDm Ti a month ago and yes there are a few things that annoy me , all of them are me being a fussy little S.O.B.. It is without a doubt the best all-round car I have owned (and I have had a few) and it is the sort of car I can see myself driving for the next 5-10 years with happiness.