Hi JFB,
I have just taken delivery of an IS220d, and its the first Lexus I have had. I have to say I would agree with many of the posts on here, yes the gearbox is "quirky", yes there is a turbo lag, yes economy does not appear to be fantastic. However that is after only 900 miles and one week.
I have been lucky enough to be used to nice cars over the last too many years to mention, (as I'm sure we all have been on this site), and in comparison the Lexus feels one of the most sophisticated, comfortable and ergonomic cars I have ever sat in.
I can feel the gearbox loosening and the engine becoming more responsive with every mile, and trust me, when it wants to go, it does go, the handling is great, the ride a little harsh but mostly comfortable. You can compensate for turbo lag with double declutch, and it does look great. For me plus' outweigh minus'.
I don't think you made a major f$&% up as I too usually suffer from buyers remorse, and its minimal with this car, but I'll post again at about 5000 miles, (a few weeks), and we can compare. I hope economy will be up, and that the low rev range has eased up and revs come more freely, it feels like they will.
Good Luck