My last car was an Evo 4, and I'm in the process of changing to an Altezza.
The Evo is an absolutely magnificent car, and I would wholeheartedly encourage anybody to have one at least once.
Pros
It's just about the fastest thing about - you have to spend silly money to get anything faster
It's glued to the road round corners
Plenty of scope for tuning - name the hp you want, and you can have it.
Lots of independent specalists for tuning (stay away from Mitsi dealers).
Cons
Built quality is fine - but nothing special. They all rattle a bit
Service every 4.5K miles or 6 months, whichesver comes first. At least £250 a time, but usually more.
You'll need a new set of tyres every year
It's bone shakingly hard suspension
It's a major target for every car thief or carjacker around. You'll need every security system going, and most insurance co.s will insist on it being garaged.
Group 20 insurance - average costs seem to be about £1k per year.
It's starting to become a Chav car (the major reason I changed)
My misses hated every last bit of it (this could be considered a pro)
I had a 4, but I've also driven a 7, and there is not a great deal between any of the models - it's a definate evolution over time rather than giant leaps. The later models are definately better, but not by a massive amount. If you go for a 6 over a 7, you won't loose much.
The source for all Evo knowledge is the MLR
MLR
For me, it was a great car to own for a couple of years, but not to stay with for the long term.
Hope this helps. PM me if you want any more info.
Ian