It should be outstanding. Subaru has a great reputation for building long lasting, extremely durable vehicles. One of the greatest things about owning a WRX is talking to people who have owned Subarus in the past. They always have a story about their diehard old Subie, running the hell out of it for well past 200k miles with no problems and selling it to someone elso who drove it for another 150k miles.
To be honest, nobody can really answer about the long-term reliability of the USDM STi. It's only been around for a few months.
The STi is my second WRX (traded in my '03 WRX - brakes too weak, gearbox too weak). The fit and finish is outstanding and the car is very well put together. I used to own a C5 'Vette with mild mods and the Subaru is built so much better. The Vette was fun but expensive to keep, couldn't hammer it in the rain, and a definite no-go in the snow. The STi feels almost as fast and should be faster with some light mods. It seats five, has a big trunk, and goes great in all weather. It has a Cd of .33 (vs .32 for my '99 Vert) and is Vette like stable at high speed - all with decent ground clearance.
It is pretty comfortable, much more so than my '03 WRX which left my back hurting after long drives. The seats aren't as comfortable as the C5 but are still really good and should last longer (I had tears in the leather after three years in the 'Vette).
It rides a little better than the 'Vette but you can still tell it has a good suspension. One thing - the ergonomics aren't bad but will not touch the C5. Once seated, the C5 is definitely more comfortable but is harder to live with than the STi.
It shouldn't start to rattle like a Vette or any other GM product after time. You said you only wan't it for the next 4 to 5 years - it should last hell of a lot longer than that. I feel that over many years it will be like an old Toyota - it will start and run good every time you turn the key and will never leave you stranded.
If you take care of it, it should last quite a long time. If you don't overboost it and it gets warmed up and cooled down then the turbo should last past 100k. The STi gearbox is pretty strong and should last but I wouldn't abuse it too much. It's AWD so there's no give and you can't religously dump the clutch, powershift, and be blatently heavy handed like you can on the T56 in the 'Vette.
I hope this helps a little. I can't comment too much because it is too new. But I've definitely been impressed with the STi thus far.