Just bought IS300h Premier (2015) and here are rambling thoughts after one week of ownership. It's the (almost) fully loaded version with Mark Levinson Audio and Premium Sat Nav.
Almost didn't buy it due to all the reviews. I've driven the same vintage 3 series and A4 and they just don't compare. The 3 series was a better drivers car, by a small margin, but are generally diesel and diesel is the devils fuel.
The IS is very smooth and refined during normal driving, no loud diesel clattering as you drive around. The engine does rev up and get loud when you floor it but you only really notice due to the lack of noise the rest of the time. There's also enough power (for me) to perform brisk overtakes without flooring it. The more i drive it, the more i like it.
The CVT gearbox is far better than people say. I drove the 8 speed auto 3 series which was very smooth for a regular auto but you could still feel the gear changes. It was never a perfectly smooth drive like the IS. Also that car had a lag when flooring it just like the IS in normal mode. (Combination of turbo lag and the gearbox hunting for a lower gear).
Another plus of the IS is the tech you get with it as standard. (We have an i3 with the upgraded widescreen sat nav, which is great, but lights for the vanity mirrors in the sunshades was an extra. Unfortunately that box wasn't ticked so we have no lights there.) I actually quite like the remote touch 'mouse' and find it quick and easy to get around. I really love how the mouse feel 'adjusts' to the buttons so you can feel the cursor going over them. My biggest issue is how slow the system is, possibly this is why people get aggravated with it. When you come from super smooth BMW iDrive it really does show it's weaknesses. Even changing songs on iPod is far too slow.
Average fuel consumption is 5.9l/100k (48MPG) at the moment and i'm not doing any fancy fuel saving maneuveres. I know a diesel might get better economy but life is too short to drive a diesel...
Thanks to every poster here who helped me make the correct decision for my new car!