I take K2, as I feel that together with magnesium, these are the only two nutrients of which it is impossible very difficult to get optimal levels of on an otherwise healthy paleo diet.
I take Nature's Plus 1,000 IU D3/100mcg K2, which provides 125% of K2 RDA. This is high enough to have an effect (studies show bone mineralisation starting at 15-45 mcg). Equally, but is not mega-dosing something which is very difficult to obtain quote Stephan Guyenet from Whole Health Source, "I can't support any supplement that has more than 1 mg K2 in natureit. That's about the upper limit of what you can get from food. Any more than that and you're taking pharmacological doses and the long-term effects are unknown."
Nature's Plus is completely free from the usual fillers like dairy, soy, vegetable oils, gluten e.t.c. Importantly, the K2 is also in the form of MK-4, not the (cheaper) natto-derived MK-7 which other people have recommended. MK-4 is the "natural animal" form of K2 which mammals synthesise for themselves. (Note: humans can actually synthesise K2 (as MK-4) from K1, but the conversion process is extremely inefficient). Each bottle also comes with a certificate of analysis from an independent lab.
K2 is a relatively novel supplement, so paying for most good quality formulations will feel like daylight robbery. I'm embarrassed to admit it, but I paid £40 for a six month supply (which I partly justified to myself as "backup D3") for a six month supplyD3"). I'm 2 months in and plan to review at the end and see whether it's worth continuing to supplement.
Results so far: I used to get staining on my bottom 4 front teeth. This is apparently quite common, and due to their proximity to the salivary gland, which releases highly mineralised saliva - something which was actually getting worse on nutrient dense paleo. Now, I've noticed that this is fading and I seem to be able to brush most of it away. This would definitely be consistent with K2's important role in dental health/reversal of tooth decay.