I've been taking a fairly high dose of fish oil (Metagenics) for several months. It was recommended by my naturopath for my arthritis and very high CRP (bad stuff for heart attacks).
I've noticed a big improvement in being able to walk longer distances without as much pain. It's about as effective as the Vioxx I was on before it was banned.
I haven't had a problem with blood thinning to date (i.e. bleeding gums, bruising), but you need to check with your doc if you are already on blood thinners or other blood thinning supplements.
I'll be getting some blood tests done next month and it will be interesting to see if it has had a positive effect on my LDL chloresterol and the CRP.
As far as weight loss - I don't think it is a factor, but I have been losing weight, partly because I'm able to be more active.
If you are interested in the subject of fish oil, heart disease, inflammation, etc. I would highly recommend a new book "The Anti-Inflammation Zone" by Dr. Barry Sears (The Zone Diet).