LOVE my Bob Revolution Duallie! Only con is that it's a bit heavy to lift into the car. But I am 5'7 and have no problems with it. This thing rolls over everything!
We are BIG BOB fans though. We also have a single Revolution.
We have taken ours to the beach many times. We just load up all the beach gear onto it and roll it over the sand without much effort. I can steer the whole thing with just one finger. I take it to the mall all the time. It is somewhat difficult to get it through the tight aisles in most of the stores, but I figure that I am saving money by not being able to get through them :). We have also taken it to a bunch of amusement parks and zoos. And on flat or paved hiking trails, some short runs, and a few 5K races.
And you can get a really good deal on it when REI has their sales (Usually twice a year). They usually go for about $525, but we got ours over Labor Day weekend last year for $425 and just had it free shipped to our local store. Then at the store they will assemble it for free.
I highly suggest that you go to a store like USA Baby and test drive all the different types of double strollers to see which one fits you and your lifestyle best before buying one.
FYI: My best friend has 3 children ages 6 months to 5 years old. She has had a Peg Perego Duette, a Combi Twin Savy (?), and a Sit and Stand. She likes the Combi Twin for short errands and hates the other two strollers she has. She bought a Bob Revolution Single when her infant was born and LOVES my Bob Revolution Duallie and wishes she could get one.
Also my kids are 20 months and 4 years old. We don't use the double as much anymore, but from the time my daughter was born to about 4 months ago we used it ALL the time. The Duallie was my primary stroller. Now since my 4 yo son walks most places and can be trusted to stay with me and not run off, we take the single revolution to the mall and for errands. We still use the double for our nighttime family walks, any long walks/jogs, races, and to go to the beach.
Other Pros:
Easy to maintain, wheels are like bicycle wheels so you can even roll over glass and there won't be a hole. Wheels are also easy to remove, you can buy a carseat adaptor bar so you can attach your infant carseat to it, you can place your infant in it directly once they are over 5 lbs in the flat position. It has a higher handle bar so there is less strain on your back, storage underneath and large mesh pockets behind each seat for more storage and little pockets on each seat for things like snacks and sippy cups. And the best PRO is that their customer service is AWESOME! I have had a few screws and pieces come off of my shades a few times and I called them and they immediately sent me the replacement parts for free since there is a lifetime warranty. Another PRO is that the resale value for Bobs is very high. I have seen used Bob Revolution Duallies go for about $400 on craigslist.
Craigslist would be a great option for you if you wanted to save some money and buy a used one. But they go fast.