They're big printers, a cube about 2 feet square. They have to be larger since the paper comes out sideways for 8.5x11 paper. It can use 11x17 paper which smaller printers can't.
We have several and about half are approaching a million copies. One is nealy 2 million copies. So far 2 or 3 have needed replacements for the fuser unit when the fuser roller surface started to crinkle or separate, so that's its worst potential problem, I'd say.
One advantage is that they were designed to work with various attachments such as staplers, sorters, etc. I've used only the duplexer, though.
If you get the HP 9000 then be sure to print a configuration page and doulecheck the firmware date. When this printer first came out 2 to 3 years ago, there were some serious problems with the new firmware. By now virtually all issues have been resolved. It seems there is a new firmware update every 6 months or so. See thread697-562332