I got rid of my beautiful 159 a few months ago, and I share some of the sentiments of the OP. I have had two 156s and two 159s over the last 10 years. Sadly I couldn't live any longer with the 159 for the same reasons. It will take a lot more than Alfa currently offer to get me back in one of their cars.
The most disappointing bit is the engine. If merc and bmw can produce 200bhp diesels which give 45mpg minimum (which is actually achievable) then 28-30 mpg from alfa is unacceptable. And god only knows where that 200bhp goes, but it certainly doesn't seem to reach the wheels! The 2.4 was a much better engine in the 156.
The handling and steering is dull and the car feels very heavy compared to a 3 series.
I don't agree about the seats. There are comfier seats out there, but the lusso seats are better than most. But they do need positioning and adjusting carefully.
And regarding reliability....simply unacceptable! OK, it's an Alfa, so you expect it to go wrong occasionally, but with my first 159, I was never sure if I'd actually get there. And if it didn't break down, I'd be sure to need a trip to the dealer every couple of thousand miles. I lost count of all the faults - manifold, dpf, egr, maf, gearbox, steering rack, electrics, etc. My second 159 was much better, but still needed a dealer visit every six months or so. And what good is it having a top of the range car when you regularly find yourself heading to meetings in a Fiat 500 courtesy car?
Then there's the dealers... say no more!
However, the 159 is still the best looking car of its class, by a long way. And in terms of equipment and comfort, it beats everything in the price range. The interior is much better than the comparable mercs, volvos, bmws, etc.
Having done 100k in two 159's over the last 5 years, I know exactly how the OP feels. It's such a nice feeling to be saving £200 on fuel, then not have to shell out another £500 on repairs each month.
The Alfa is a good car in many ways, but it is expensive and inconvenient to own one. Other makes do have problems, but if you break down once in three years with a Toyota you'd count yourself unlucky!
I know this board is intended for Alfa fans, so I hope I don't offend too many with my views as an ex loyal Alfa fan!