http://www.fieldlines.com/story/2008/3/21/15257/8068

Rainwater great.
If you live out west make sure you have water rights.

NO to reverse osmosis system.
The last thing you need for rain water is reverse osmosis system.

Yes it can remove many things, not sure if rated or accepted by code for bacteria/virus which is what you need for rain water.
If you dump the first of the rain the water is quite clean, important the roof is metal or tile. Water must be sterilized.

Besides the ultraviolet (UV) consider ozone to sterilize the water.
The UV lamps need replacing yearly, ozone should list decades. Uses less power.
Ozone can sterilize the whole tank, UV just the water flow in or out.
Check out these links for more information:
http://www.arcsa.org/
http://www.harvesth2o.com/
http://www.irc.nl/page/29189

http://www.puritec.com/store/category.cfm?Category=76

Using ozone is a cheap and inexpensive way to kill the bacteria and most viruses.

There is a cheaper way that most public water supplies in the US use. Chlorine.
Also, when private wells are tested and lab test indicate there is bacteria in the sample, bleach is normally recomended for treatment.

You can buy a gallon of bleach at just about any grocery store. Get the cheapest, unscented stuff you can find. If you are concerned about putting too much bleach in, buy a chlorine test kit made for swimming pools.

You can email me if you have additional questions or I will post here. I can get you or anyone all the info they need. BTW, I'm not trying to toot my own horn but I do have a lot of experience in the water/wastewater field.

Roof washers (still find that a strange name for water catchers or diverters) are needed for a drinking system. They can be anything from a fancy measured capture system to a valve you turn on and off by hand. A screen or screens to keep most of the big chunks like leaves out pipes and tanks is a real good idea.

If you are using a pressured delivery system then you need a screen filter before the pump. A charcoal filter (taste and chems)and a small/hole size micron filter(dirt or pollen) after the pump or pressure tank. Filtering can be done on just the line/faucet you use for drinking water. I knew of one family that for the first few years filtered their rain water for drinking through a Brita pitcher system. If you are using a gravity flow system then ask your plumbing supply house for suggestions on inline filters or use one of the countertop filtering pitchers.

The two main choices for disinfection (there are others) are UV and chlorine both have their niches. UV on an automated pressure system is great and leaves no residue in the water. This also means that it gives no protection to any of the rest of the system and takes power for the pump and the UV unit. A UV sytem set up for batch disinfection is where you have a UV light that shines in the holding tank for a set amount of time on a set schedule. All the UV systems I have seen were automated. Chlorine is cheap and easy if used as a batch setup but there are many automated chlorine systems out there. It is the main chemical used for public water supply systems. IF you are going to use chlorine(bleach) please be careful and know your math and chemicals. It can be as much of a problem having too much chlorine as not having any. Caution it likes most metals and will eat up a tin tank. It also tends to leave a residue when it combines with organic material. Luckily most of the normal concentrations of chlorine and residues are removed with the charcoal and micron filters. I like a combo setup the best where you have an auto UV and do batch chlorine 2-4 times a year depending on local conditions.

A rain water system can be as easy or fancy as wanted or needed. I grew up with a 55 gal rain barrel that caught water from a roof valley. It was simple and easy. Take the lid off when the water coming off the roof was clear and put it back on when it quit raining. It was what we made coffee, tea or cooked beans with. How good your rain water is depends much on local conditions. How often and what type of rainfall, trees, roof type, dust, pollution in the air from nearby factories and your rain water setup including holding tanks, filters and plumbing all affect water quality.

Enough preaching I did not read the links posted but bet they have all or most of the info I just posted.

http://www.water-research.net/ozone.htm

Short story is: various types of electrical activity (lightning, electric motor running) can produce ozone which is O3.

O3 is very unstable and quickly degrades into an O2 which is very stable, and an O1, which is highly reactive.

The O1 aggressively attaches to iron, sulphur, bacteria, and other water-borne items. This causes the chemicals to form particulates and fall to the bottom of the water tank, or causes the bacteria to die due to reactive cellular damage.

Do not drink ozonated water until it has rested for at least 30 minutes to ensure all O1's have reacted, which typically occurs in less than 15 minutes. Ozonated water should be transferred to a holding tank for at least 30 minutes before being consumed.

As a side note from other research, De-Ionized water has some uses, but drinking it will cause a slow health degradation. When consuming DI water is stopped, health will slowly return with no permanent affects.

The R/O systems depend on charcoal filters for removal of chlorine and like chemicals.

All filters, R/O or any other, need to have the particulate material removed prior to the filter unit. For 20 yr., we used rainwater off a steel roof and stored in a 2200 homemade cistern [concrete block on a concrete pad and lined inside with a cistern liner, which can be ordered custom made from many sources...for our big tank, a new one last yr. ran us $350 I think it was]. You WILL get a lot of dust, dirt, dead plant material, insects, etc. wash off the roof into a cistern or barrel or any other storage. The biggest challenge I had was figuring out a good and efficient way to filter those things out. I ran gutter downspout and elbows into the cistern, from the roof, so the 1st line of defense was simple screen material at the downspouts to block the larger stuff. I had to clean out the gutter and screens every month or so.

Then the water ran through some filter medium I set in a concrete box I built on the side of the cistern, and the water then ran into the cistern from there. Then, our intake line to the house supply, 3/4 inch poly, had a 1 micron screen on it. Then, just before the pressure pump, we have a simple, inline filter to make sure we got everything we could, as any tiny thing in the water can damage those pressure pumps.

Then, we had a carbon-filter unit at the sink with a separate faucet unit for drinking and cooking water from that....like most of those units come with these days.

We had to go to a well last yr. [legal reasons in our state], and just hooked the well line into the cistern and bypassed the old filter stuff as the well water is extremely clean. But, it's hard, so the reverse osmosis system.