The only negative thing I've ever heard about flaxseed (and I've been using it for many years, way before I heard of low-carb) is that, as Natrushka mentioned, it is very perishable--particularly the oil--and when it oxidizes it is not good for you. This is why the oil is always packaged in dark bottles and specially sealed, and once you open a bottle you should keep it in the fridge and use it up very quickly. I always worry so much about the oil going bad, that I just use the seeds now (which can also go bad, but if you do as Natrushka suggests, and buy the whole seed and grind them yourself, you really shouldn't have to worry. This also tastes the best, since it is so fresh. For conveniance, though, I use Bob's Red Hill Flaxseed meal in a pinch, and yes, it is available in the U.S. now--in Manhattan, anyway).
Also, another benefit of the seeds is they are a GREAT source of fiber--much more gentle and effective than psyllium seed, I have found. When I used to have a constipation problem (on a high-carbo, high fiber, vegan diet, I might add! ) flaxseed helped alot. Also a great source of other nutrients, not just fat.
Also, when you're low-carbing, flaxseed meal is very useful in making hot breakfast cereals that substitute well for oatmeal and such--I eat flaxomeal every day!
Enjoy!