Re: buying 323?
The Mark X and the IS250 are different cars altogether. Built on a different platform, transmission, different chassis, body panels, instrumentation, light clusters, NVH insulation etc. The only thing they might share is the 2.5L V6 engine. Don't think we can compare on cost alone, as it would be like comparing an Audi and a VW equivalent (e.g. an A3 2.0 FSI vs a Golf 2.0 FSI). Both are from the same manufacturer (VW-AG), sharing the same engine, but are not the same car. However, in the Audi-VW example, at least they share the same platform.
I believe there was a deliberate attempt by Toyota, since the point when it officially launched the Lexus brand in Japan in 2005, to segregate the 2 marques substantially enough. I guess with time, that divide will become more significant. From a business standpoint, I think it is just inevitable, if Toyota wants to raise the benchmark, perceived or otherwise, in its 'relentless pursuit' of developing the Lexus brand beyond what it already is today.
On that point alone, I respect this Japanese company for giving the world a strong contending alternative in the luxury and performance car segments. Gaining from their (and Honda's) strong push / return to Formula-1 racing, and getting to the podium more often than not these days - even displacing the Ferraris, BMWs and Mercedes teams at times, I think the world can only benefit from the Japanese re their rapid advancement in engine design and reliability improvements.
As for looks, I think we are in a very subjective area here. To each his own, I would say.
I recall Winston Lee in his IS250 review 2 weeks ago having said that when parked side by side, he thinks the IS250 will take the honours for looks from the 3-er.