18s slowed my acceleration down by quite a bit....its a really noticable difference whenever I change from stock steelies to 18s. Even modding in the performance category didn't help much to compensate for my heavy a s s rims. BTW, I didn't get the lightweight expensive kind, I saw a deal and jumped on it w/o thinking too much. The car handles a lot better and cornering is improved vastly. Oh yeah, I have a 6g I4 sedan non-vtec and in stock form it comes with 14" steelies...(maybe thats why I notice a big difference in accelerationh.
Even with all that said I'd still suggest that you get them. It'll make a world of difference in how your car looks.