Graduated filters

I'm still on a shopping spree ever since I acquired the D90.  I have just ordered these six 72mm graduated filters from eBay.  There is no gray filter though.  They are red, orange, green, blue, purple and yellow.

While searching for graduated filters, I ran across adapters for those square Colkin filters.  I think Colkin filters are really great.  You can reuse the same Colkin filters for any lens as long as you have the proper adapter for the lens, and they are really cheap (less than $10).  I was so tempted to buy the Colkin adapter and filters but I remember using them in Cuba a few years ago.  At 18mm, the adapter is too deep it creates vignetting in the corners.  I have three lens with a 72mm filter ring so I thought these circular filters will be fine.  These happen to be the lenses I usually carry around, and hence, these 72mm filters.