Welcome to the DIS boards and
DVC. You should really start off your quest for DVC information up at the top of these boards where there is a great "DVC FAQs" section. That said, I'll take a quick stab at your questions:
1.) You can make your DVC reservations at any time of year. The "point cost" will vary by DVC seasons and is listed above on the DVC
Point Charts. In general, the charts will not change over time (there is no point inflation, so your points today will pretty much buy the same thing in 10 or 20 years). The "Use Year" you see in the listings relates simply to the time of year that you will receive another year's worth of points, per your contract. If your use year is February and you have a contract for 200 points, you'll receive an additional 200 points on February 1st every year through the duration of your DVC contract. Those points must be used by January 31st of the following year (or banked into the next year according to the banking rules). When you receive the points has nothing to do with when you can make reservations.
Even though you do not actually receive points until February 1st, you can make reservations for those points as far back to March of the prior year (11 months earlier) -- to make a reservation for a February stay using that year's points. When making a reservation, DVC just cares whether the points are available at the time of the resort stay, not at the time of the reservation. (Does that make any sense?)
Reservation rules are a separate topic -- and you should review the FAQs to see if you can get an understanding there.
2.) The only current theme park discount is 10% off of the length-of-stay passes during your DVC visit. You may also drop (at your option) the first and last day of your trip from the tickets. However, one of the benefits of DVC is that you can stay a bit longer and enjoy things outside of the theme parks. Many DVC'ers opt for annual passes or discounted park hoppers from wholesalers, depending on how many days we intend to be on property in a given year.
3.) Maintenance fees vary by the resort you buy into (as your "home" resort), and generally range from $3.75 - $4.25 per year, per point in your contract. These rates can change from year to year. They can go up and go down -- and have historically done both. Over time, you should expect them to rise a few percent over time as costs for upkeep and resort employees increase for inflation.
4.) It's a personal choice. If you are a "Disney" family and enjoy going once every year or two, DVC will likely increase your enjoyment of your family trips. You'll be able to stay at a deluxe resort in a large apartment style accommodation (most with a full kitchen).
For my family (4 kids) it was a no-brainer as we no longer have to book 2 separate rooms. We are actually saving a small fortune over booking regular rooms -- and we'll save even more by cooking some meals ourselves. Of course, the flip side of that is that we are likely to go a lot more than we otherwise would have done without DVC.
5.) Points can be "banked" for 1 year, meaning you can save up any amount of your allocation from one year and push it to the next year -- but they must be used in that subsequent year. Points can also be "borrowed" from the next year in the future if your don't have enough points in the current year to book your entire stay.
6.) You can absolutely bring guests with you -- or just make a reservation for guests to stay on their own. It's your place. You can even rent points that you cannot use in a year to someone else that wants to go to a DVC resort.
Again, my strong advice is to read-read-read. This website is a great tool. Here is another synopsis of DVC that I found useful before making the decision to buy:
http://www.mouseplanet.com/dtp/dvc/1_Forward/forward.htm