I use it, and have been using it for a while with Bugsy.
Which one to choose depends on what you need it for. Some of them are better for increasing the immune system, and some are better to modulate the immune system, which means to increase or decrease it accordingly (in cases of gingivitis and allergies, for example).
First of, here is where I get mine from, and from all the shopping I have done, it seems to be the best deal:http://www.transferfactor-us.com/tra...-products.html
The strongest version is the Transfer factor Plus - ($56 for 60 capsules). This one should be used for increasing (considerably) the immune system. The capsules are a bit large, but the contents don't have much of a taste, so if your cat is not too finicky, it should be easily "concealed" into their wet food. This was the one I used for Bugsy for quite a while... 1 capsule a day is what I used.
The next version is the feline. This one, even though it seems less expensive, in the end it turns to be much more expensive, as one can will last about 15 days for a cat like ours. You also need to mix a lot of it in the food - like 2/3 tsp, so it does add considerable taste to the food. The smell is a sweet sugar cane smell, and IMO not really enticing. I would not recommend this version.
The last, and the current version I am using, is the TF Classic. This is the one recommended to modulate the Immune system, which is what Bugsy needs, as he has immune mediated gingivitis. It does increase the kills cells, but it also decreases the self immune response. This one is recommended by vets for immune deseases, such as allergies, gingivitis/stomatitis, etc. It is also the most affordable one; $37.95 for 90 capsules. The capsules are much smaller, which makes it easier to be accepted by mixing it in their wet foods.
Since I started Bugsy on TF, he has not gotten sick, which is a BIG deal. However, this is not the only supplement he takes... I am sure it is part of it though...
I have also taken everytime I was coming up with a "bug", and it has worked like a charm - having it at home has kept me from getting sick more than a few times.
I hope this helps.