ive owned a 1990 4.5 liter and now own a 1997 N* deville with 118k. my parents own a 99.
ive only had my N* for a month or so, but im loving it thus far. my parents have had the 99 for over a year, its got about 75k on it. no problems with my car thus far, knock on wood. my parents have replaced the EGR valve, alternator, and plugs and wires. repairs are very costly on the northstar, and illumina is correct, the average driveway mechanic will have a very tough time doing anything to it.
to compare...an alternator died on my 4.5 deville and i replaced it myself for 150 bucks. a couple weeks later my parents did the alternator in the northstar at the dealer...500 bucks. see what im getting at.
from what ive heard the 4.9 is as reliable as any other engine out there. of course, each has its own set of downfalls.
do your homework on whatever car you decide to buy, namely in the tranny and coolant departments and test drive both. to me the 4.5 felt more gutsy for some reason, i think cuz it makes its power down low. the northstar definitely hauls, but its like its TOO smooth or something lol.