Update: 150k and still going strong. Leaks a little oil from around the pulley covers (end of the engine block), but KA24s do that. In all it's about a pint every 3000 miles, so it's hardly worth pulling the engine out to fix.
Valve cover gasket replaced, cured leaks at the top end.
Front sway bar links replaced, $12 a side, front end quieter, and pulls nice and straight under braking.
Replaced an EGR control solenoid (caused a CEL, Haynes and OBD scanner to the rescue), got a used one online for $10, so easily fixed. More CELs and bad MPGs (down to 20 MPG) caused by a dirty connector on the intake sensors, cleaned with brush and alcohol, back in business and 24-28 MPG.
Just about to get more tires, last set have gone 30k.
Fitted an aftermarket CD/MP3 head unit that takes USB and SD cards, front audio jack, remote control, face/off unit, just $70 at Target, vast improvement on the stock radio, even with the stock speakers. Trying to resist temptation to add 6x9s or an amp; after all, it's a 12 year old car I'm busily trying to destroy.
Replacement (reman) driveshafts fitted at 130k working fine, showing no problems or CV boot splits after 20k. Time to flush the auto trans again, it's had about 30k since the last, but fluid is still clear and it shifts smooth, Lubegard HFM ATF supplement noticeably improved it and helps mitigate for using Mercon III in place of Nissanmatic D.
What will I replace it with when it eventually blows something expensive or is damaged? Probably an 02-on Altima 3.5 SE; prices are dropping now the revised '07 is out.
Update: Now at 192k, the knock sensor needed to be replaced for the classic P0325 OBD code ($25 online). Due to the seized bolt holding it in, I needed to pull the passenger drive shaft for access, and since the CV boot was beginning to split/rot/leak, I decided to replace that too; found it at Advance Auto for $50. Looking further up this thread, that's 8 years and 60k since it was replaced. It's hard to feel too mad about $50 in parts.
Finally got around to replacing the front crank pulley seal. A FelPro kit for $8 including the RTV, should slow down the oil drips for a while. Add an oil pan gasket for $6 and it's dry for now. Same for the transmission pan, another $8.
It's a daily driver, and still gets around 26 MPG average.