18 " really only good for a willy waving contest or if you mainly drive around a McDonald car park with your stereo up loud trying to impress fat girls with corned beef legs in mini-skirts!
i fitted 18s on my 156. i agree with the sentiments here, that 17s probably provide the best compromise between ride comfort, handling, weight, and cost.
but it's not all terrible with 18s. it certainly looks good, and i found ride comfort very acceptable here in australia (where our roads are not renowned for being overly smooth!). i had 225/40 tyres (pirelli P7000), and did upgrade the suspension to an Eibach Pro Kit/KONI Sport combination.
now, my willy will be massive after i fit 19s on the GT (!); car is booked in for today for fitment.
seriously, however, 18s are quite livable in my opinion. but have the suspension sorted as part of the upgrade. the KONI dampers made the biggest difference, ironing out the 'crash' into potholes and ruts.
over the longer term, account for the added expense of 18" tyres vs 17". the car deserves quality/performance tyres, so don't go 18s only to fit the cheapest, nastiest tyres you can find.
if you live in an area which dictates winter and summer tyres, you can use the stock wheels for winter tyres, and have 17s/18s for summer.