Maybe Peleg can jump in as he just sold his STi. I had a 02 WRX (non-STI) and it was a lot of fun. However, it was a bit rough around the edges. It had a lot of rattles, the stereo wasn't very good and I had clutch judder problems with mine. I'd definitely take a long test drive and make sure you will be happy with it. They definitely aren't luxury cars.
08 Smoky Granite Mica IS F
I had a 2004 STi, which was the first USDM STi, it was also the lightest in terms of curb weight (3260 lbs). Those cars are a lot of fun in terms of grip and power. The AWD is awesome, especially in wet conditions.
Power wise, the new models got more reliable turbochargers (the old ones had a faulty designed wastegate, which used to crack), the 300 horsepower factory stuff is just the beginning, with an ECU, downpipe, fuel pump and tune you'd be just about 400 bhp on a car that weighs about 3350 lbs.
The transmission is great and can handle a lot of torque.
The STi (2008 and on) did change quite a bit from what I had. The interior got better, the ride a tad more supple. I will say the factory seats suck balls, I upgraded mine with a pair of RECARO's from an 03 Evo, so if you're serious about cornering, consider this one of the best mods.
Compared to a Lexus ISF, the STi might be equal or less as far as rear seat room. The interior as whole will not be as spacious, the materials will not be as good, might have more rattles here and there.
Engine wise, you get half the displacement and half the cylinders, however the gas milage will be about the same as an ISF. Reason for that is the engine is turbocharged and those snails like to suck in a lot of air and demand alot of fuel from the ECU. Ofcourse that car only takes premium fuel.
On the freeway in 6th gear, 80 MPH you'll be doing about 3500 RPM's. The engine does not sound or behave like a typical 4 cylinder, the turbo kicks in very very quickly and the engine has a sweet torque curve down low. Like I said, with a Stage 2 pro tuned package (About $2000 at the most), you'd have enough power to play with a car like the ISF down low, and walk away from IS 350's, bmw, mustangs etc.
The brakes are great, Brembo 4 piston up front, 2 piston in the rear. I upgraded the lines and fluid.
On the inside, DEFI gauges for boost and oil pressure and a NAV system add up a lot to this car.
Best thing to do is go drive one, and then jump on www.iwsti.com, I have a lot of friends there who will be happy to give you their opinions.