i got a real world 20 highway mpg with fresh tuneup on a 65k mile 99 3.55 geared car.
i was not impressed. it was a fun car to own for a week. the 4.6 just lacks the balls (torque) that a heavy car needs to be fun. it was a comfortable ride, commanded GTFOMW(get the eff outta my way) from other drivers, and was not bad in the winter with altimax arctic tires. on the firestone "cop tires" it was crap in real winter driving conditions.
i saw a 6.0 LS swapped one once, i would totally rock that. i'm sure i'll do it one day too.
check frame of 98-02 cars very carefully directly below the firewall for rot, mine went there by 75k and needed scrapped.
and no matter what some people say, i have never seen a difference in quarter mile times from any stock p71 i've ever seen run from 98-08 so don't buy the "omg you need to get one with PI heads or you are slowwww" crap floating around the internets. it's just that - crap. you're slow no matter which 4.6 you get.
just my .03