To be fair, that thread contains only 47 posts with 138 votes, so it doesn't really represent the majority of Lexus owners and an IS220d isn't really what you would describe as an enthusiasts car, so like you say the majority of those owners have likely joined that forum because they've experienced problems. Toyota easily put more than 138 IS through the workshop each month and occasionally you'll see more than that in a single week, so I'd take it with a pinch of salt.
Personally I wouldn't spend my money on an IS, but that's not from having any quality concerns, they just don't float my boat. Although it is fun abusing an IS-F every now and again.