I have 18s at all four corners and love them. There are two critical components though, if you're going to go that big: stance and wheel design. You have to nail the stance with wheels that big otherwise it'll look wrong. Big wheels need a lowered car (not ridiculous, but lower than stock). As for wheel design, that's a personal preference but you need something that goes well with your overall look.
'66 Coupe - Pro-touring in the making
289 or 302? 4bbl
9" TrueTrac 3.55 gears
TCP suspension front and rear
18x8 Wheels and Baer 6P all four corners
And no more money left in my wallet...