I'm on my third one. I think they are exceptional cars, and the reputation as being expensive to maintain is way overblown. I am very active on the rennlist 928 forum, and you should remember that these forums represent only a small percentage of actual owners, and ussually are the guys (and girls) who like to do their own stuff. As such, it may seem like there are lots of issues, but there really aren't. They are much easier to work on than the Boxster - you can actually see the engine! Also, for the entire run, the 928 was the flagship Porsche - the fastest (except for the limited offering of the 930), the most expensive and the most luxurious. This means that they were more complicated than the contemporary 911s (altho the modern Boxster is more complicated than the early and mid-model 928s). They were way ahead of their time, and still look current. They handle beautifully - it is a different drive from the Boxster.
Lastly, don't forget that 928s are older cars now. And like any older car, things need to be replaced and maintained. Because the prices are cheap, maintanance tends to get deferred. Always look for the best one you can afford, with a good service history. This goes for any car of this caliber.
If you want to see a good write up the last month's issue of Hemmings Sports and Exotics features a 928 in the Buyer's Guide section, and it goes through a lot of good information. Oh yeah , btw, the featured car is my current 928 (shameless plug

).
I am new to this model and I am smitten. I love my Boxster. I bought a good one just a few months ago, but I did a couple of things already - it was overdue for a plug change and air filter. I did the oil and added the LN spin filter adapter, I had to replace a headlight bulb, and now one of the microswithes in the driver's window is faulty and I need to replace the lock mechanism. I say these things not as a knock against the Boxster, but as an example of the fact that any car that is getting older will need things. The Boxster parts are no chaeper than the 928 parts. The forums are what make the difference to me - between this forum and Pedros garage, I have a wealth of knowledge to tackle whatever the Box throws at me (ther bird - Pelican - is pretty good too; Rennlist, which is where I spend the bulk of my forum time, is horrible for the Boxster).
Anyway, my $.02.