I have 2 children, 6 and 14 months, and one due in July. When my son was a little guy my neighbors gave me their Burley. It was for 2 and it worked great. They took the trailer back when my son got bigger. We wanted to get a new trailer for the 2 new children so we did a lot of investigation and shopping. We learned that Burley is among the best, and yes, they are pricey. As we have been shopping we learned that Burley changed ownership and the newest models are not made as well. I can't be more specific, I think the bottom line was that they were using cheaper parts. I have a book called baby bargains that suggests buying a used burley to save money. They sell them like hot cakes on ebay. Also, you could buy one at a garage sale or I think people will bring them into bike shops.
We ended up buying a bike trailer called the Chariot. It is expensive but it is nice. THey sell them at REI. We will use this bike trailer as our double stroller. It's advantage over the burley is that the ball joint that connects something to the bike rotates so that if you wipe out, the trailer won't tip with your bike, it will stay flat. The chariot also converts to a stroller, a jogger, a hiker and if you can believe it, you can attatch skis to it if you plan on snow shoeing. All of these extras cost extra.
We LOVE it! It was expensive. They sell them on ebay as well.
My baby bargain book suggest both of these carriers and they say if you are planning on using the trailer often, spend the money. If not, a target costco whatever brand will be fine.
Hope that helps
M. K