This is an interesting topic because I considered both cars prior to purchasing the IS 300. My opinion was based on large amounts of research and being a previous Maxima owner ('91 SE w/5 speed).
Service/support: Both cars offer excellent performance and value, but our purchasing decision was based not only on the car, but the entire purchasing experience. The Lexus approach to car sales and after-purchase support is excellent in comparison to the local Nissan dealer. At the Nissan dealer, you can expect long lines to get service along w/no loaner cars. At Lexus, loaners are always available and lines are non-existent.
Engine: The Maxima has an excellent V6 with power to spare. The power is a little "peakier" than the IS. The IS has a smooth engine that does not "feel" as powerful as the Maxima, but gets you to speed just the same. The Maxima for ooomph and the Lexus for smoooooth. We put over 100,000 miles on the Maxima engine, so we know it is a quality engine made to go the long haul. The longevity of the IS has yet to be determined. The engine on the Maxima is perhaps its strongest point. What a great V6.
Suspension: This is not close. The Lexus is well balanced and far above the Maxima for both ride comfort and handling. The ride is firm, but still maintains that comfortable Lexus ride. The relative softness of the ride belies its ability to navigate the twisties. The Maxima is not as refined in its approach to suspension design.
Gearbox: This one can be debated to no end, but the wife just got tired of driving stick so we chose the IS. Even if the IS had a manual transmission, we still would have chosen the auto w/e-shift. Plus, the e-shift is fun.
Exterior sytling: This are is very subjective. The IS is the sexiest car I've seen in a long time. My reaction to it occurred on a visceral level. The styling of the Maxima appears to be more practical. It's not ugly, just plain. My brother on the other hand felt the IS was to "plain" and that the car that did it for him was the M3. To each his own.
Interior styling: Hands down, the Maxima has the best guage cluster I've seen. Easy to read, practical and only the necessary guages appear. The IS guages are too busy.
Overall: The IS just fit our CURRENT needs better. We wanted a sporty 4 door sedan that was automatic. It is a safe car w/many safety features that are not present in our old 91 Maxima SE. It also was RWD to allow me to return to my driving roots (I used to drive a 85 Corolla GTS that had RWD). The IS is not perfect. To find that, look no further than the current M5. In the future, I intend to modify the IS to allow the engine to put out between 250-300 hp. Just my opinion.
P.S. Excellent site, informative.