I lost circa £9500 on mine - 2007 (57) 2.4 SW Lusso having owned the car for 43 months. I figure that wasn't bad. In real terms I got around £1500 more for a 6 1/2 yo 159 than I did for a 2.4 156 at 4yo. Sure... I could have got more had I hung out for it, but when I decide to sell I want it gone, the car gets priced realistically (I use Glass's as a guide) and all 3 of our Alfa's have found new homes within 1wk.
The issue with selling an Alfa is that you generally need someone who wants an Alfa, not someone who is looking for a a saloon/estate (as with the 159) who is open to any marque as long as the car ticks boxes; whether we like it or not, whether it's our experience or not, one of the first comments when you say you drive an Alfa will usually be "when are you getting a reliable car?" or something along those lines. The wider perception is one of poor build, suspect reliability, dodgy electrics, etc. May not be real, but that's still the opinion of most.
...and I still can't believe Alfa have nothing to offer me, other than the same car with TI trimmings and a less characterful engine. I'll always look enviously at the 159 but I may never return to Alfa having turned to the Darkside.
... I've not gone for a new Beemer, a used 6-series instead, but the deal was good and 3 year unlimited mileage service plan is just £600!