I've owned 23 cars in the last 13 years. That's about two cars per year, although I swear it doesn't seem like I've gone through them that quickly. I've owned my Baja for almost 18 months, and it will have to be pried from my cold, dead hands.
Yes, it will hold a 21x21 box in the backseat, although the backseat is definitely smallish (it's the same size as the Outback and other Legacy's).
Yes, the engine does seem a little under-powered, but I think a lot of that will also depend on where you live and how you drive. I'm fairly demanding, and drive pretty steep grades regularly, and I can get what I want out of it, but sometimes have to push it a bit. On the otherhand, I've driven my Baja to the midwest and found the power to be more than adequate, maybe even peppy at the lower altitude. Check out cobbsport.com for performance accessories.
As far as MPG goes, in mixed driving I too get 21-22, in and around SLC. But, when I did drive from Utah to Nebraska I did manage 28! That was at 90mph, 100+ degrees, windows down much of the time, A/C pumping. Oh yeah, and the temperature gauge didn't move.
Cargo...I've hauled two fridges (laying down), concrete, furniture, all kinds of crap. Obviously, noone is going to buy a Baja to go to the construction site, but if you are realistic about what you are getting, I think you will be impressed. A previous post mentioned the bed extender taking up room, but you shouldn't worry about it. The extender comes out very easily, and will fit at either end of the bed, takin gup little room. It also fits in the back seat.
Don't, DO NOT, buy the Subaru soft tonneau cover. It's crap. Pure crap. If it wasn't on my car when I bought it, and had I actually bought the cover on my own, I would've been pissed.
I don't know what else to tell you. As you can probably tell, I am quite pleased with my Baja. It's uniqueness and versatility are tough to beat. I don't know if you have your heart set on a brand new one, but I bought my 03 at Nate Wade Subaru in SLC (natewade.com), with
2,000 miles for CHEAP. At the time, the same money would've gotten me an Outback that was twoor three years older, with at least 50,000 miles on it. Every so often Nate Wade gets 5 or 6 2003 models, with virtually no miles, and lists them for 18,000ish...negotiable.
Have a look at scoobytruck.com for other info. Good luck!