I do it all the time, but I add the ground flax seeds. I too have heard that it is hard for us to use the nutrients efficiantly when in shell form.
The shake gets a little gritty, and the last slug is usually full of it. Plus it seems to be a good idea to swirl or shake the drink often between sips or else the last slug gets really gritty.
I have found that it works better in milk than in just water.
If it is mixed in with fruit it becomes a lot less noticable. Alllla - smoothie
I have also noticed that Chris Shugart did the same with his V-diet. I tried that diet, hence the reason I know that skim milk works better with the ground flax than water.
My local grocery store (piggly wiggly)carries the brand Hodges Mill and they sell ground flax seeds for like 1.60 a box, pretty cheap. I also found it at a local health food store for about the same price. Just make sure you keep them refrigerated after you open the bag. They also sell wheat bran which comes as little flakes, I like to sprinkle that in my oatmeal. Almost pure fibre. However very bad in a protein shake.
On a side note I have seen places start selling Flax-Oil pills too, however you wont get the fibre benifit. I actually like the fibre in the seeds, there are like zero carbs in the product that are not fibrerous.