Raw milk is a controversial issue. Personally I'm a huge advocate, but everyone has to make their own decision.
If you plan on getting raw milk from a farm, I can equip you with some tools to help you source clean quality milk.
One website to check out is A CAMPAIGN FOR REAL (RAW) MILK!
The most important thing with all milk (regardless of pasteurized or raw) is to be sanitary. Unlike beer, there are many pathogens that can survive and grow in fresh uncultured milk. Potentially dangerous bacteria are present everywhere. Staphylococcus lives on your skin, and Listeriosis lives in soil, to name but two. They are present in small quantities in ALL dairy products (believe it or not) including raw milk, and quite a number of other food stuffs including both animal and plant products. These bacteria are harmless at the levels they occur normally, but if they find the right growth conditions without competition can reproduce exponentially, to dangerous levels.
Even non-dangerous organisms are an issue of concern. Would you really want to drink milk or yogurt that smells like vomit, or that has a yellow yeast or black mold growing in it? The organisms that cause these problems are not usually dangerous to your health, but they are also not desirable in most circumstances.
Fresh milk (regardless of weather it is pasteurized or raw) is an excellent growth medium for many different organisms. It is rich in carbohydrates (lactose), nitrogen (proteins), moisture (water is the primary component of all milk), and it has a nearly neutral pH (about 6.7).
Simply because a milk is raw or pasteurized (there are many misconceptions on both sides of the isle) does not make it immune from contamination by unwanted bacteria. Even the best quality fresh milk, if it comes in contact with a sufficient population of unwanted bacteria due to uncleanliness, will become spoiled and/or dangerous in short order. On the other hand, even poor quality fresh milk, if it is treated and handled properly, can be kept free of unwanted bacteria by diligent dairy processors.
My goal here is to help you get you the best of both worlds: High quality fresh raw milk that is handled properly and diligently.
Once the animals have been milked, the milk should be chilled down to 32-38 degrees F immediately in the farm's bulk tank. The milk should be kept very cold until it either is consumed or is intentionally cultured with good bacteria. 40 degrees F is the absolute highest the fresh milk should (ideally) ever get while being stored. 33-35 is preferable.
Once you get the milk, it should either be consumed fresh within 5 days, or cultured into yogurt, kefir, cheese, etc... to make it more stable
When culturing milk, make sure you inoculate it with desirable bacteria every time. This could be as simple as a tablespoon or two of any number of cultured dairy products -- including live-culture store-bought yogurt, yogurt from your last batch, sour cream, kefir grains, etc... -- as long as you are confidant that the culture is alive and healthy, and is not contaminated itself. You can also get freeze-dried pure cultures shipped from Dairy Connection Inc. (just google them)
Simply letting raw milk "culture" on its own is like playing Russian roulette. Some people do this because they think that once the milk has become sufficiently acidic (pH below about 5.0) only lactic-acid bacteria can reproduce. They are right. But in the time it takes for this to happen, bad bacteria can also reproduce exponentially, and can continue to survive even after the pH has dropped below 5.0. This is even more of a problem if the milk has been improperly handled, and so there are higher levels of bad bacteria to start with.
Also, be aware of the cleanliness of the container and transfer vessels you use for the milk. If your empty jar has any off-smell, it is not clean.
Make sure your lids are thoroughly clean and replace any rusted or scratched lids. Make sure the lip of the jar and the thread for the lid's ring are free of debris and dried caked-on milk. Jars and lids should be washed and scrubbed thoroughly with warm soapy water, then rinsed with hot water. It is also recommended they then be sterilized with either bleach or boiling water. Running them through a dishwasher on a high-heat cycle also works.
The jars and lids should then be AIR DRIED. (Drying with a towel is only re-introducing more bacteria.) Any standing moisture in a jar is going to become a growth medium for bacterias and molds if it sits around for more than a few hours, so make sure your jars and lids are completely dry.
Finally, don't be afraid to ask some hard questions about the farm's animal husbandry practices.
Start with an easy question: What breeds do you have, and how big is your herd?
Are the cows on pasture? If so, do you use a rotational grazing system, or is it just a single pasture paddock? (rotational is better)
What percent of their diet is fresh forage during the grazing season, and what percent is hay? (Hay is good, but not as good as fresh grass and clover)
Do you feed them any sillage? (Sillage is bad... but if they only feed it during the winter just don't buy the milk during the winter)
Do you feed any grain at all (it is not uncommon to feed them a small amount during milking... but too much can be a problem) Do you feed them soy? (Soy is bad, but a small amount during milking is acceptable)
Questions about the milk:
Do you test the raw milk for coliforms and pathogens? How often? (once every month or so is OK, as long as the results are coming back negative)
What is your typical butterfat and protein percentage? (useful for cheese making and estimating yields -- a high ratio of butterfat to protein is better for soft cheeses. For harder cheeses you want more protein so it might be necessary to skim some of the cream if it is particularly rich)
Hopefully, I've provided some resources that are useful to those seeking out raw milk. If you have any questions, let me know.