I made an impulse purchase the first time -- I was staying in an All-Star room, cramped with my DS3, and my parents (one of which had an
ECV). Since due to my son's age, I needed to spend time in the room, it was horrible. When I saw how much more room there was in a 1BR, I was sold on the spot. I don't regret that purchase at all.
I have also added on 5 small contracts and now have 460 points. It's enough for two trips per year in a 1BR, especially since one of those trips is at peak time. If I sat down and really ran the numbers, I probably wouldn't have bought the last 100 points. Now that I am planning these trips, I am realizing that the cost is so much more than the cost of the points. The dues, airfare, park tickets, food are all a huge part of the expense.
We have two trips in 2010 and another two in 2011 (one of which includes a 3-day cruise on the Dream). Park tickets are already purchased, so my only future expense is airfare for one more trip (the rest are already purchased.) Since I will be borrowing points for the final 2011 trip, we will likely only take one trip in 2012, or may even bank those points for trips in 2013. The airfare is ridiculous and driving is not an option.
I have spent A LOT of money on top of the initial cost of the membership. That monthly deduction out of my checking account for dues is a big one. This is all something to take into consideration.
Having said all of this, if you can afford the purchase, dues, airfare, park tickets, etc., for a couple of trips per year while still being able to pay your normal living expenses, I say GO FOR IT! It is wonderful to be a DVC member and fun planning future trips. We haven't taken our December trip yet and I have already planned, booked, bought plane tickets, and even made ADRs for our April trip. I will be booking our November 2011 while we are at WDW in December. It makes me so happy planning our trips.
All in all, I'd say that your decision to buy should be primarily a financial one. If you are a Disney lover, the enjoyment part of it is just about a guarantee.
Good luck!