Why not Evo5?, easily better looking than the 6, but the Evo6 is the ultimate Evo, especially in TME form out the box of course. Evo7 onwards have been refined a little, but they are still very quick and capable machines.
Reasons for not having one.... you worry about it every where you park it, infact, you worry about it just driving it! You will lose your licence very quickly, and you need shares in Shell and a second morgage for up keeping, they require servicing every 3k miles.
Reasons to want one... any Evo will pretty much be the quickest point to point saloon money can buy, and driven properly, they are so much fun, if a little to clinical. You will get bored of 280bhp, and find a stage1 remap will give 300+ easily, happy days.
You also won't care about 15mpg when you are giving it the beans.
All just my opinion of course, I'm lucky enough to have driven and test drove many Evo's and Scoobys, and whilst they are a hoot, I can't see myself living with one in the long term.
Of course, I still have my S3... but that's only because I can't afford the Audi R8 I want to replace it with... yet!