What I found kinda funny is that if your at 7% and by the Manhatten wonder and 20 nukes you can push yourself into the 5%
I myself have bought one, it doesn't really slow down growth all too much (For me it was like 13 days on a vacation come back and buy one) But buy like 4 economy wonders first though, the quicker you buy 'um the more work they'll do for you.