First of all, these rebates seem to be very good for certain lenses. But, it varies by lens. The best thing to do is to look at the price history charts on canonpricewatch.com and see if the lenses you are thinking of buying are discounted enough to make it worthwhile.
The two lenses you mentioned were the 50mm f1.4 and the 50 mm f1.2. It looks like the f1.4 is only about $20 less with the rebate than what it was selling for pre-rebate.The f1.2 lens looks to be a better deal, selling for about $100 less than pre-rebate. (But it is a dang expensive lens in the first place.) I note that B&H is selling the lens new for less than they have a refurbished version listed at (a pretty good sign that it's a decent price.)
In either case though, you'd have to decide if the savings are sufficient to justify the hassle of buying the lens in the grey market and getting it home.
By the way, if anyone needs a rationalization for buying Canon lenses, this is a fun article:http://www.canonpricewatch.com/canon-lenses-better-stocks/