I love my 928. I find it no more expensive to maintain then my 911. It's super smooth, fast (especially from say 40 on up). Thing is you have to drive them, find one that hasn't had the electrical hacked around with...these cars have alot of electronics in them and have miles of wires. The engines are rock solid and run forever (they never really work hard). The weisach rear end is a work of art and really makes this thing handle unbelievably well (51-49 weight distribution I believe).
Key thing, like the 944's and 968's, is to perform regular maintenance per factory recommendations on the timing belt and water pump. The 32V engines and euro 16V engines are interference engines. If you get a 16V US version they are non-interference, thus if the timing belt snaps you are just stranded. But these cars, if maintained like any other car, will go forever. I've put about 60k miles on mine in the past 4-5 years and have never been stranded. Just performed regular tune up stuff (oik, filters, plugs) and a timing belt/water pump change. Had the alternator rebuilt once for 160 bucks.
These cars are cruisers...I drive back and forth from Milwaukee WI to South Bend IN every weekend and it's like I've only been in the car for a few minutes..it is that comfortable. Thing you have to watch out for is the speed creeps up on you. If you are not paying attention you'll be in triple digits before you know it (and don't go by the engine sound cause it isn't screaming at 100!). Remember, these are 160mph cars (oh, and you can have your sunroof open and still hear the music over 100!).
All I know is I will never give mine up (oh, and my wife likes it the most out of all my porsches).
To get the skinny on 928's go to:
http://forums.rennlist.com go to the water cooled are then choose 928's. That board is as active for 928's as this is for 914's.
Another plus of 928's..they were extremely expensive in their day (last year 95 models had a 150k dollar sticker price) but they are super bargains now. I picked up my 83 with 62K miles on it for $6000.
Just be leary about one with extremely low miles or one that has had alot of patching on the electrical.
Jim