Josh, you know my thoughts on the 300M so I won't harp on that too much here. As for the 300 being an improvement over the 300M, I have a different opinion on that as well. I've driven the 300C and 300C SRT-8 and while they do have more power, I prefer the 300M. The 300M handles noticeably better, and while it's subjective I prefer the overall driving experience of the 300M a bit more.
One thing to consider when you're looking at the reliability ratings. Toyota and some of the other brands here make fine cars, but it's still often true that parts cost less for American cars. For me, the 300M strikes a very good balance of performance and luxury, with a lot of bang for the buck on the used market.
Just for fun, here's a link to a video one guy recently made of a fun day with his 300M on the track.
http://www.85merk.com/SRT300S/dunnville/11aug07/