I have a 156 2.0 JTS Selespeed and the engine is amazing and the gearbox is very smooth. I'll admit it wasn't for the 1st week until I got used to it, but after that it is as smooth as any manual gearbox and a lot more fun to drive.
They aren't as bad for reliability as they make out, basically it is a standard 5 spped gearbox with a hydraulic actuator to control it all. They changed after 2002 to a more reliable system. When you have the foot to the floor and rev's over 5K, it goes into fast change mode and it is way faster than any manual box to change and you don't have to take your foot off the accelerator pedal.
As for service costs, Alfa specialists are great, £99 for a minor and £175 for a major and they really do a good job.
The 2.0JTS only has 1 spark plug per cylinder and in standard form is 165bhp which is 15 bhp up on the twin sparks. Cambelts are definitely something to watch, the tensioners were plastic on older models and will fall apart causing the belt to slip and destroy the engine. Later models and also replacement parts are all metal tensioners and these are much better. It's about £320.00 for a full belt change with new tensioners and balancer belt.
Unfortunately I am selling our 156 as I need an estate, but they are great value for money. Our's is a July 2003 (03) 2.0JTS Selespeed with factory options of Xenon headlights, Sat Nav, Momo leather sports seats, Bose speaker system with sub and a load of other bits. It's done 45K miles from new and has just had it's first MOT. I expect to get £6,500, which for a £24K 3 year old car is real value for money.
Don't be put off by the reputation, a post 2002 car is as good quality as the majority of cars on the road. I am just ordering a new Saab 9-3 Vector Sportwagon and I can honestly say the Alfa feels better built with higher quality trim.
Although I am aged (i.e 36), the insurance on the Alfa is only £300 through RAC.
Buy one, you know it makes sense.