My problems with Nokia N900 device started only when it was already out of warranty; nothing major, yet. So I cannot vote about Nokia customer care, but I can say that their build quality is good. As good as it gets, in modern world.
The Service Manuals indicate that any disassembly-assembly of a device involves replacing many parts (screws, some other fragile bits and pieces) with new ones. It should indicate high quality of their repairs, even if their customer-catering skills are quite poor.
The latest (version 8.0) Service Manual for N9 was printed on 20.07.2012, if I remember correctly. It alone indicates ongoing support for N9.
Best wishes.