well, coming from an alfa 166 to a lexus GS300 i feel both unbiased and best qualified to comment here..
first, (cirtainly for the flagship 166 anyway) some myths...
rust. no.. not for a long time, my 2000 model didn't have a spec on it, paint finish was notably better than the lexus too, far less prone to stone chips (both cars are within 1000 miles of eachother)
electrics, nope, not in the slightest, everything worked perfectly.
build.. awesome, in the 166 (dont know about the lesser models) its easily equal to the lexus, the door shut sound for example is definatley better, and nothing was loose or fell off..
reliability, hard to say, mine suffered a couple of very major expensive failures, all down to past neglect, but the opinion on carsurvey.org is the 166 is a fantastic car. will i own another? definatley, its a fantastic drive, responsive comfortable (better seats than the lexus) well built, its not as quiet or as quick as the lexus, but the key is how its been looked after. treat it as a mondeo, and its going to break, treat it like a baby maserati, and you should be fine.
i really like the lexus, and i have greater confidence in it, but i am going to be really sad to see the alfa go.