Everybody drinks water, right?. We all know we're supposed to get our eight glasses a day. So, why not keep your brand in front of your customers, while they keep themselves healthy -- not to mention environmentally-conscious. If this is the only "eco-friendly promo" you do this year, it's a good one. It'll be used (therefore, your logo will be seen multiple times) every day (and what more do you want from promotional products?); your customers will love it (and use it!); and you'll be doing good.
This is a winner.
Green your brand. Start here.