We own weeks at a couple of resorts in Cancun - we love it there although it is currently a little far to travel with a two year old.
For us, the pros of owning are:
1. The quality of the exchange resorts available - usually 4 or 5 star resorts.
2. The price which we effectively can stay at them (i.e. our stay at the MGV effectively cost $98 per night which I calculate as follows - ownership annual maintenance fee $450, timeshare exchange fee $120, exchange company membership $120. Total $690 for 7 nights in a 2BR) Of course, if you bought at a resort and returned to the same place every year, you would not need to worry about the exchange company fees. I believe Marriott operates their own exchange program within the Marriott Vacation Club.
3. You know you are guaranteed your 'home week' each year. This can be a real help if you travel often to the same destination during peak times.
The cons of owning:-
1. The high initial cost of investment. You will note I have conveniently left out any of the initial cost in my calculations above. This is money I have written off, but if you are considering buying you will need to factor it in. The general advice is that there are plenty of good timeshare resale opportunities to be had at approximately 80% discount to retail, but it is a long time since I bought mine so things may have changed.
2. You need to plan well in advance to secure the best exchanges for peak times (NOTE: for many resorts peak times are 52 weeks a year). It is not unusual to have to plan 12 - 15 months in advance. Fortunately, Orlando is usually available at much shorter notice due to the huge number of resorts in the area.
10 years ago and living in the UK, holiday prices were high particularly for transatlantic vacations. Timeshare offered good value at the time. The advent of the internet has changed that and the benefits have diminished. We have stayed at some wonderful resorts through the exchange program - much better than we would have normally booked - but, I am not so sure I could justify the initial investment today with the availability of cheap deals for travel to the US. In Europe, many hotels still charge on a per person basis - if you are travelling with 4 or 5 people, timeshare generally yields bigger savings here.
Hopefully that gives you a small taste of what I get out of it. Ownership is a complicated and individual decision, just make sure you are not pressured into anything - I was once, and was kicking myself for years.
Andy